[Archrms]

I can't tell for sure because of the picture quality, but it definitely looks like some type of shrimp, possible a mantis shrimp. Typically you are supposed to quarantine your live rocks in a separate tank for awhile to ensure there are no "hitchikers" like this. Be thankful it is not a fire worm, which will eat your expensive live rock. Check with your local shop or one of the mail order warehouses, I believe they have traps for them.