Looks like a decorator crab (Schizophrys sp ?). They are #1 on Calfo and Fenner's list of Worst Bet Crabs due to specialized diets, decorating themselves with bits of polyps, algae, mushrooms, etc. Not trustworthy with inverts and small fishes due to carnivorous tendencies.
Sure looks neat, though. Maybe you can take him back to the store where you bought the rock or keep him in a separate refugium or FOWLR.
na, it's not a decorator crab. i've seen a few of those at local stores.
i threw him in the 'fuge already anyway..just incase. this guy is not much bigger than the head of a pencil eraser.
my refugium is like the amazon jungle. i throw emerald crabs in there, and i see them for a day or two, and then never again.
he has the same chance as everyone else- KILL OR BE KILLED! 8O