Looks like a rock crab to me. Their typically nocturnal. I don't think there is anything wrong with keeping them in a reef tank. Someone else may be able to answer that.
I was told they clean your SPS but one day I bought a new SPS and that came along with it. All my SPS started to die. I dunno if it was the crab doing the damage or something else.
I'd get rid of it or banish it to the sump to be safe. I'm fearful of any crabs except hermits, pom pom, and porcelain. Just my opinion, but better safe than sorry. You'd hate to wake up to see him eating one of your fish.
brianjfinn;605099 wrote: I'd get rid of it or banish it to the sump to be safe. I'm fearful of any crabs except hermits, pom pom, and porcelain. Just my opinion, but better safe than sorry. You'd hate to wake up to see him eating one of your fish.