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He looks like an algae grazer by the flat tips on his claws. My LFS has several black "emerald mythrax" crabs in their coral tanks that aren't for sale. I'd just keep an eye on him. Although ok while small, most all crabs have the potential to become a nuisance when they get larger.
I've left him alone because I suspected that he was an emerald crab, but his color is brownish black, so I was not sure if that was a color variation found in that species.