Mithraculus sculptus
The Emerald Crab, Mithraculus sculptus, is an herbivore. Also called the Green Clinging Crab, they do a great job of eating algae, including the dreaded bubble algae. These, like other crabs, need a good calcium level to grow well. Some people will say that they are "opportunistic", but I haven't had any issues with them bothering any other inverts or fish in my tank. They do get a little large and sometimes inadvertently knock over coral frags. I have seen them wave their claws at passing fish, but I think this is just the emerald crab's way of defending their turf... Reef safe in my opinion.
