How long have you had it? You said it is eating regularly, does it eat a lot? How often are you feeding? If it is eating a lot already but getting thin it may have a worm parasite and would need to be treated. I think Prazipro is the treatment of choice for that but you may want to do some research.
If you go to the ARC search box and put in keywords of "parasite worm" or "prazipro" and hit search it will come up with a lot of threads that could be applicable. If it is a worm, I'm not sure how to know this for sure, but if the fish is getting thinner even though it is eating well it would seem to indicate that. Prazipro is a reef safe medication that has been recommended in the past that may be the route to go. Read some of the threads and hopefully someone else with more experience will chime in.