A large (1" dia.) crab came with my live rock. He is mostly black with marbly-white on his arms. He is very shy (can't get a picture - yet!) and stays in the porous rock. Occasionally comes out. He seems to pick stuff off the live rock and also responds to shrimp pellets placed near his 'lair'. He has 2 large claws which he sometimes uses to sweep water toward his mouth. I have only 2 fish (4 stripe damsels) and he seems to shy away from them - they will try to take the shrimp pellet out of his grasp! My tank is 3 weeks old - cycled with live sand/live rock. 55g, CPR Skimmer, Refugium, Ammonia-0, Nitrites-0, Nitrates-5, Alk
-440, Temp-77F. Any idea of what he is?