It is a type of mithrax crab. The rounded cupped claws provide the ID on this one. The green crab, Mithraculus sculptus, is part of this same family. Not all mithrax crabs are good to have in aquarium - most when small are fine b/c they will pick at algae. All crabs are opportunistic scavengers. Some prefer algae, but when there isn't algae to eat they will go looking for something else to eat.
My opinion - I'd keep it for now, but I'd watch it. If you have a sump, that would be a good location for it to scavenge around for food when it gets a little bigger. Since you are cycling this crab might help to keep some of the algae that you will have growing in check (if it survives the cycling process).