I would pinpoint when exactly it happened, which fish was introduced at that time? Tangs are very susceptible to stress, blue tangs will fade to an almost white when stressed, mine did when I first got him. But feeding good food, and removing the stress factors will hopefully get him back to healthy.
The garlic is a good idea too, I feed mine garlic infused seaweed sheets.
It can boost their immune systems and help them fight off stress.