purple clawed crab

[waynefish]

Hi, I've noticed a hitch hiker crab in my tank. He is small, about half the size of a penny. He has purple claws and he seems to be eating coraline or the rock. His legs are spotten, with little bristly hairs. He kind of pinches the rock and brings it to his mouth. I've tried to take pics but he is too quick. Any help would be appreciated, if u can just give me a name, I could try to ID it on google. thanks a lot.