Looks like a little pistol or snapping shrimp. I have 3 that hitchhiked on my live rock they don’t cause any problems... but they can be noisy sometimes when all 3 are popping. I say keep it. Where did you get your rock from? Gulf? Keys?
I have one in each tank, they came as hitchhikers. The one from KP aquatics makes loud pops and makes a sand pile next to the rock where he lives. The other came from Australian live rock, makes less noise, and no sand pile.