It tends to be low pH or soft water that will take the shine from shells after a single boil. Shells have a high pH of around 10, but in some parts of the US water can be as low as 6. Anytime something with a higher pH is exposed to a low pH the minerals will start to dissolve. (Which is why it's unavoidable with substrate, since cocofiber is around 5.5 and sand is 7.) Soft water which has less dissolved minerals will really want to steal the minerals that are in the shells, and heat and the movement of boiling will speed the process up. Using salt while boiling will also etch the surface of shells and cause them to dull pretty much instantly.
Scarlz, tap water in George tends to be around 7.0 pH and is rather moderate on the hardness scale. Compare that to my tap water here, which is a pH of 9.5 and a hardness that is almost three times as much. I can boil shells for a half hour without any damage to them , but a month of the crabs wearing them on the substrate will make them look ancient!
The reason why it's etching and dissolving and not a deposit of minerals from hard water is because you can't grind or polish off the ashy look to get the shells back to new. If it was just something sticking to them, then a fine polish would restore them. Instead if you were to look at them closely you'd see the pitting and wear on the surface, and polishing would just wear it down further.
And if anyone wonders why our pretty hermit shells fair the worst - even natural shells have been cleaned using chemicals when they're being sold for any human purpose. since humans don't want snail pieces and stink in their decorations or whatever. Couple that with any polishing that destroys the outer protective layer and the shells start aging at a much faster rate then they would if left alone on a beach somewhere.
The ashy look to the crab is the same thing that is happening to the shells, since their exos are almost the same composition. It's why they molt continuously throughout their lives, so that they can repair that damage as it occurs since it's unavoidable - if they couldn't molt, they would die when their exo wore out. When a crab starts looking that bad, it means it's time for them to molt. If it's been a while looking like that and they still haven't molted, it's not a bad idea to reassess their tank to make sure that the substrate is deep enough and that it's comfortable enough for them to want to molt. Some crabs put off molting for a long time so it's not really something worry over, but it's something where it doesn't hurt to make sure everything is okay for them.
