take a red flashlight or red LED light and shine it in your aquarium and find him. Find out exactly where he hides and then take that rock out and put it in FRESH water,... he'll evacuate. Either that, or if you're really desperate, get some heart worm medication from the vet and treat that SPECIFIC rock in some saltwater and wait overnight. (the medication only kills crustations) Make sure you wash off your live rock very very well and quarantine if before adding it back into your aquarium. Good luck, happy reefing!