HCA: Hermit Crab Association

My crab, Hermie, was active and seemed happy, while we thought our other crab, Coral, was dead up in a moss pit. She is fine, but Hermie is now under a second pile of moss on the sand and I have heard some digging. He hasn't been out for four nights. (I didn't know not to put moss directly onto sand until recently and he hasn't left it to add a container to the moss.) How do I tell if he is buried and molting if I can't see him? I don't want to take Coral out of the tank because she finally seems to be getting comfortable. I don't want her to be a threat to him either if he is molting.
Some advice on would be appreciated. Thanks!

They may very well both be molting. My perspective is to leave them alone as much as possible. Unless you have an obvious surface molt with another crab up-and-around that may eat the molter, I'd just maintain your conditions as good as possible, keep providing good water sources and good food (particularly including good sources of chitin and protein so whoever gets up first is not as tempted to eat the other one) and just leave them alone.

Coral may be molting in the moss pit (that happens often, especially for weak crabs that aren't strong enough to dig much, or in crabitats where the substrate is not deep enough or a good texture for making a molting burrow). Hermie may very well be molting under that pile of moss. If you are worried that he may basically be on the surface just under the moss, instead of dug down into a proper molting burrow, you might want to make an in-tank "iso" spot around him.

In the meantime, gently smooth out the sand every night, everywhere except right where Hermie is; then you can see if Coral (or Hermie) is actually moving around at night. Sometimes you don't see them for days (or weeks or months), but they can still be very active in the middle of the night. If you smooth over the sand before you go to bed, you'll see "crab tracks" in the morning where they've been.