Hitchiker crab, good or bad?!

Got a crab, most likely bad and it looks like a black mithrax crab. It is really dark purple, there was two but one seemed to be eaten up. It came with one of my sps corals.

I would like to keep this guy but if he is really bad for fish, corals and other inverts than he will be banished to the fuge

 

It is an acro crab, will not harm your fish as they stay small. They feed on a few of the polyps on the coral and anything that sticks to the corals. Generally a beneficial relationship for both.

 

thats good news, I kinda like the guy. He does scurry everywhere at night, as soon as the lights on on he hurrys back to the birdnest coral lol which is pretty cool