Hitchhiker crab.. killer?

The tank has been going for about a month and a half now and today I noticed a small crab in the live rock - had to be a hitchhiker. I did not get a good look at him and unfortunatley an even worse picture (time for a macro lens!). He looked to be dark grey in color with lighter grey bands on his legs. He had what looked like a pretty sharp claw which looked to be brown. So far I have had a fire fish and a peppermint shrimp (bought two - found one dead - have never seen the other one) die (or murdered). I also at times hear "cracks" coming from the tank - like the glass is cracking... freaked me out when I first heard it. Is this guy a suspect or an innocent bystander?

 

The cracks can be mantis shrimp or a pistol shrimp. I'm sure you'd know if you had a pistol shrimp. Mantis shrimp are a little harder to find and they would have ot be pretty darn large to kill your fish. I would suspect a mantis is what you have and the cracking would be it cracking a shell. As for your crab, I can't tell... If it's fuzzy and has serrated claws then I would get rid of it.