I have 2 emeralds and one is a bit greener than the other. I've read several threads about emeralds turning white. No one seems to know why or how to get them green again. Possibly just problems with diet or low iodine levels? Anyway thats just what I have seen.
I recently got a white hitchhiker crab on a rock with shrooms and kenya trees. It looked alot like a emerald except it had smaller black claws. The emerald I have had more wide claws and the ends are not pointed. I now believe that the little hitchhiker is a gorilla crab. Check the claws on your white crab and see if they are black and pointed.