It looks like a type of xanthid crab.I had one many years ago that love snails but never bothered the fish,shrimps and other crabs.Toss it in the fuge,if possible.
i think reefreak might be right it has hairy legs and scoops for claws like its a cleaner . i dont know but my snails do fine so he is not eating them. he only comes out when i feed the meats.