I once had a "white" yellow tang shipped to me from my supplier in Hawaii. Paid a good deal for it as it was supposedly collected that way from the ocean. We put it in the section with the rest of our tangs. After about 2 weeks of continuously eating the Tang diet we made special for the store ( high in greens and vitamins) as well as Romaine lettuce, which was the lettuce of choice at that time, the fish turned back to it's beautiful yellow coloring.
So in your case, make sure you have good water quality and highly nutritious foods for your tang. It may be suffering from nutrient/ vitamin deficiency from being in a tank for that long. Also be aware of any bacterial conditions that may be taking over on your fish. Since Tangs don;t really have scales, their skin is more susceptible to skin issues. Maybe a picture of the fish with a close up of the area may help us better diagnose the situation.
Hope this helps.