its a rock crab...just judging by the hitchhikers guide sticky...not really sure of the specific species...they usually just eat whatever they can get ahold of...not sure if they actually eat corals....he was just stealing some smaller frags(more or less just the plugs)..and using them as protection i guess....i have a huge black mithrax crab in my tank too....but they are "usually" fine....i speared a smaller one of these guys with a skewer a while back....my wrasse went nuts!!!!.....pretty funny