if u have live rock in the tank, its probably a hitchhiker called a mantis shrimp.
Cool little buggers, but only if you want one (you probably dont). They kill almost everything u put in the tank including fish, inverts etc. Your going to need to try and locate him, and remove him.
Watch your fingers when doing so, as they can create pretty harsh lacerations.
The way i would recommend removing him is finding the piece of LR hes is located in, and take it out of the water. Put it in a bucket full of either freshwater, or water very low in salt (probably a bit better for the rock) and he will come flying out, you then remove the rock and do what you will with the mantis.
Many people keep them in separate tanks, because they are so cool to watch and own. I wouldnt kill the mantis, and im sure many people here wouldnt mind taking it off of your hands...
good luck,
Jay