Looks like a ruby (aka strawberry) crab. They're supposed to be peaceful, but a crab is still a crab and will eat whatever it can get a hold of. They're supposed to be reef safe, but I've read about multiple people getting rid of them cuz they were eating corals. It's a nocturnal crab; I have one and rarely see it. Although it is a suspect in a fish death, I didn't see it do anything. Others have had suspicions of theirs eating fish also. As I said, a crab is a crab- it's opportunistic.