Your lobster is a pistol shrimp. Look them up on the internet. They live in crevices or sand caves under the rock. One pincer is much larger that the other. You maye hear a "popping" sound at night when he pops that claw. It is used to stun prey with the sound wave that big claw makes when he pops it. Lots of people buy them for their reef tank. He is a "good" reef inhabitant and may help by eating bristle worms.
Mike