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Looks like an emerald crab. They have a bad rap imo. They will eat algae in your tank. But remember they are scavengers and opportunists, if you dont have enough algae your corals will be on the menu.
I always thought emerald crabs to be green for some reason. He's got pretty fuzzy legs too. I've got quite a bit of algae but once I notice a difference I'll trap him and get rid of him.