i would try to catch the little bugger and give him a freshwater dip..always seems to work wonders for the fish at our shop!
there are also a couple products out there that claim to be reef safe treatments for ich.. one is called Kick Ich, made by Ruby Reef... I have used this product with some success, but still find that fw dips are more effective, however, when i did use this as a treatment, i did not lose any inverts/corals!
the other product is called exodin and is made by Aqua Medic... i havent used this one yet, but the parents of one of the guys that i work with at the lfs used this on their tank to treat a couple tangs with ich and it cured them, but it also killed several corals, namely a giant bubble coral, and a torch and frogspawn...like i said though i have never used this one...
i would also try adding a garlic supplement to any foods that you feed the lemonpeel, as garlic helps to stimulate the fish's appetite, and also helps to prevent ich in the long run...it might not cure it right now, but if the fish gets better, and you continue to feed him garlic soaked foods, his immune system will be much more prepared to fight off such a parasite!
good luck
jon