You can scatter some seeds (chia seeds are super easy) right on the moist EE substrate, and they will sprout. The crabs will eat the sprouts, very good for them. Of course, this isn't the same as growing long-term, established live plants, but it's a super-easy way to start.
I've grown live plants in my tanks for years. I could not get air plants to stay alive (you might; I don't know why they always failed for me). A couple of different kinds of fern species lived for a while, but always eventually browned out and died (possibly the "salt air"; not sure). I even tried a little orchid once, in a tiny pot attached to the side of the tank, but that didn't last very long at all.
What IS happy to grow in a crab tank? POTHOS. When I was gone for months last year taking care of my parents in Florida, my husband kept the crabs alive but never watered the plants, so they died, so I am bringing up new ones to put back into the tank. Here's what I do:
Buy small pothos from local Lowe's/Home Depot/anywhere that sells house plants (if you can find a source for truly organically-grown pothos, you could by-pass some of this, but even then, if it's been sitting near plants that HAVE been chemically treated, you still might be adding pesticide residue to the tank, so patience is key, anyway). I bought mine around Christmas, since I knew the crabs would likely be emerging from molt around now, and once they were all up, I could mess around in the substrate and put in new plants, etc.
Take the little pothos plants out of their pot (there will usually be several plants growing in one pot). Rinse the soil off their roots. Put them in a new pot, with organic potting soil. I separate them and put them on different sides of one really big pot, for convenience, but so I'll have two or three plants to put in the tank eventually. Let them grow in that pot, establishing more roots and processing out any pesticides, for about three months. I'm paranoid about chlorines, so I water them with bottled spring water. That's probably overkill for the whole three months, but I would definitely not use plain tap water for at least the last couple of weeks before you plan to put them in the tank.
To keep them safe while they establish in the tank, I buy small (either the 4 or 6 inches in diameter) terra cotta pots (you can get these cheap at Lowes/Home Depot, or at Hobby Lobby, which sometimes has sales on potting things and they are REALLY cheap then). For the 6-inch pot, the hole in the bottom is usually just about the right size to cram a cork from a wine bottle in to stop up the hole. For the smaller pot, I take some aquarium sealant and seal up the hole (do this outside, the fumes are highly toxic), then let it dry for several days outside to let the toxic fumes die (after several days it is fully "cured" and completely safe to use in your tank).
I use terra cotta pots because they are GREAT for slowly dispersing moisture (from watering the plant) into the surrounding substrate. I block the holes in the bottom because if you don't, then the amount of water you have to use to water the plant will flood the bottom of your tank (found that out the messy way!).
Once the pot is ready and the plant has processed out any pesticides (it has grown in organic soil for at least three months), pot the plant in your stopped-hole terra cotta pot with organic potting soil; make the top layer (about an inch or so) with your substrate right out of the tank. This helps discourage them from digging (because if you just have organic potting soil, they will REALLY be interested in digging in that different soil, and eating it!), but it isn't enough. I also find some creative way (different with each pot) to make a kind of grid over each pot, which the plants can grow out of, but the crabs can't fit through. For example, I put a plastic rocking horse I found in a flea market (I have a "whimsy" theme for my 90g) over one of the pots. The base of the rocking horse is made in such a way that it forms a grid between the slats of the base of it and the rim of the pot. I pull the plant's stem(s) up through holes in the grid. I previously rested a piece of heavy climbing decor on top of the rocking horse to keep it there, but this time I think I will just use zip ties to secure the rocking horse to the pot, so I don't have to put something on top of the horse if I don't want to. You can also make a grid by cutting larger holes in plastic needlework canvas for the plant's vines to come through, cutting it to fit the size of the pot, and cutting slits around the edge for zip ties to attach it to a zip tie ring about the pot (below the rim, there is always an indentation that you can use to put a zip tie "collar" under). Sorry I can't remember how to post pics here, I'm sure that would help. But the main idea is to make a kind of barrier to keep the crabs out of the pot, so the plant has its own little "safe space" to grow from, where its first, main roots are defended from digging.
After that, sink the pot into the substrate so the top is about level with the substrate around it. You can also just put it on top of the substrate if you like, and the terra cotta will disperse moisture into the air, but since the crabs cannot climb the terra cotta sides of the pot, if you do this you lose that space, in a way. I do have one pot like this in the 90g, but I have a cargo net strung over it so that they can visit that "level" (the top of the pot) if they want to. It also adds to the visual obstruction in the tank, which is good for them psychologically (they don't like feeling exposed), so if you have a big enough tank, that works, too. Sometimes I nestle a food shell in there; I try to vary where I put food so they have to be more active to find it (that's a basic environmental enrichment technique zoos use for captive animals; it may help them stay in somewhat better condition in their cramped captive situation).
So, either sink the pot or use it as a visual obstruction and possible small bit of "level." Then remember to water it now and then (use whatever fresh water you offer your crabs; remember that your tap water has chlorine in it so do NOT just use that). I often dump the dirty fresh water (or at least some of it) into the plants when I take it out to clean the water source. I figure the crab poop is fertilizer for the plants.
Do not use your MSW; salt water will kill the plants. (Along that same train of thought, plant your plants near the freshwater/ "jungle" side of your tank, rather than right next to the MSW source.)
Then watch your real vines grow! Once they are nicely established, they will send out LONG vines all over the place. You can wrap them around your climbing structures, and they will quickly give you real leaf cover throughout your tank. Where they touch the substrate, they will send out roots, and some of them (if they find a place that isn't popular for the crabs to dig) will establish and strengthen the plant (EE with crab poop and the occasional spill of leftover worm castings makes pothos perfectly happy). Once they are happy growing in a tank, the only problem may be that now and then you have to pull some out because they are taking over. An additional good thing about live plants with roots in the substrate is that, if you do over-moisten the substrate by accident, they will help you absorb the spill. It's just a good thing in general to try to have as much of a dynamic, interactive mini-ecosystem (with both "producers" and "consumers") as you can, within your means and the parameters of the tank environment.
I don't know why the crabs do not eat the pothos; but they don't. Having live, healthy plants in your tank is SUPER for adding humidity to the air (they give off water vapor from their leaves), and of course helps to freshen the air and provide "live" oxygen for the crabs as well. I highly recommend it, it's worth the time and fiddly-ness. It not only makes their environment healthier ("live" O2, additional humidity, more leaf cover for less stress, more natural surroundings), but it's beautiful, too. Best wishes creating your own little living jungle for your crabs.
