Sorry to hear about the continued harassment and having to remove him, as well as the possibility of ich. I'll pass along an LFS owner taught me several years ago, who has been in the business of saltwater only for over 30yrs. If a fish is still eating mix a paste of a frozen food, such as mysis, garlic guard, vitamins and metronidazole ( i always use Seachem). Feed once a day for 7-10 days and you should cure your problem. I would also feed this as a prophylactic to the other fish in case it had time to spread. It is safe to use in a reef and will not harm any inhabitants. Directions call to use it with Focus which is a binder, but i have used without in a pinch without issues.
I know your pain of adding a fish only to find harassment. Purchased a Blonde Naso and my Fowler's is being the aggressor. Is making me extremely upset right now, but I have a couple of plans in mind. Just seeing which one would benefit the Naso more before pulling the trigger, so to speak.
Good luck and keep us updated!