Hey Jason,
Do you have any algae in you tank? The natural food of Emerald Crabs (
Mithrax sp.) is algae, and that is what gives their bodies that nice
green color. If they do not have enough algae to eat, they beging to lose that color. That does not mean that they are necessarily unhealthy. Just that they aren't getting enough of their natural diet.
If you don't have much algae in your tank, you could get some
Seaweed Selects at your LFS, it is dried seaweed, pressed out in sheets. Just get a quarter of a sheet, get it wet so that it is pliable, then put it partially under a rock or something. I'm certain that your crab will
TEAR IT UP.
HTH
