If you have a quarantine tank, I would get the fish and place it in there and start treating the fish. If you dont have a quarantine tank your only optionis to do a couple things.
1. Go to your local fish store and get garlic, soak the fishes food in it everytime you feed. This wil helop boostthe fishes immune system and help to fight off the parasite.
2. Treat the tank with a product called Green X. I have used it in my full blown reef of 3 years, with all types of corals, starfish, shrimp etc. It is totally reef safe and will kill the ich parasite.