Crab ID please...?

[fishbait]

Hello,

This morning we were watching our cleaner shrimp molt and all of a sudden the two clowns and a bi-color dottyback started attacking him and bit off his eyes and now he's dead!!

Anyways, due to our closely watching the tank we noticed a little red crab (see pic) that is about 1 cm across running around in the tank. I can guarantee you we never put him in here, so I guess he was in the rock or something before we got it. We captured him for the time being - can any of you recognize this little guy, and better yet - do you know if he is beneficial or harmful for a reef system? I figured if we put him back in we may never see him again so we should find out first if he's good for the tank.

Please note the pic is close up - the crab is only 1 cm across.

Thanks