Definitely a pistol shrimp. It's easy to miss them as they can be quite cryptic. He may be small or have a burrow at the back of your tank. I see dozens of them on good live rock shipments all the time. They're harmless. If it's a species that gets bigger, you may be able to match him to a goby when he gets bigger. 
		
	
		
		
		
		
			
				__________________ 
				AI Man 
James 
Aquarium Illusions
			 
		
		
		
		
	
		
		
	
	
	 |