The emerald crab is an emerald green marine crab which is also know as the Mithrax crab.
Emerald crabs are often kept in reef aquariums to control algae, in particular bubble algae as they live on a mostly vegeratian diet. Emerald crabs usually peacefully coexist with other reef inhabitants until they decide to snack on other animals with those huge pincers. Emerald crabs also can grow quite large for a reef tank if they manage to live long enough.
I have bought the specimen in the pictures recently to help combat the algae still growing in my reef tank after a major disaster has left it with a high level of nutrients and plenty of algae growth.
I never had any real problem with mithrax crabs before, but as usual only time will tell if this particular crab is reef safe on the long run.