He is not an emerald crab (Mithraculus sculptus) IMHO. The claws are wrong and also they have smooth claw apendages vs the very textured ones in the pics.
I would say it’s a close match to a xanthidae, probably one of the gorilla crab species. Difficult to say what he’s feeding on. Could be algae, could be snails or coral. They are always a risk and not considered reef safe.