Hi, glad to help. Do you have a quarantine tank set up? How long have you had the fish? If you don't have QT tank, you're going to want to get ready for a fresh water dip asap. Fill a bucket half way with filtered water. Make sure the water is the same temp as the aquarium. You're going to want to dip for about 3 minutes. Let me know about QT tank and fish history and ifbit has been eating.
You're dealing with a bacterial infection. The fresh water dip may be enough to clear it up if the bannerfish is well established and was eating well. I would still set up a qt tank and prepare to do an antibiotic treatment. A basic 20 gallon with hob filter will work fine. First let's get that dip done asap and get a little relief for your fish friend. I imagine you might have had a PH shift during your last water change. It happens. This is a common cause of stress and immune dysfunction in fish. Some are more sensitive than others. If not treated it could develop into pop eye or just stress the fish beyond the point of recovery. Lets see if we can avoid that!