What type of setup do you have, FO, FOWLR or reef? And what is the size of the tank. What are your water readings, particularly nitrates and phosphates?
When is the last time you did a water change?
My first suggestion would be to do a water change. Many times this will improve what appears to be sick fish symptoms. Additionally, what are you feeding the fish? Tangs are mostly vegetarian. If you are feeding it just lettuce, that will not do. You will need to get real seaweed products such as SeaWeed Selects, etc.
Do you have a hospital tank? If water quality and nutrition is not the issue, then you could be dealing with a bacterial infection which will need to be treated with antibiotics.