The whitespot parasite is present in most reef tanks, waiting to attach to a stressed fish. Tangs are particularly prone to it as they don't like being moved around. Often people simply get whitespot on their tangs because they not acclimatised the fish properly.
How long have you had the fish?
What are your water levels including Ammonia, Nitrite, Nitrate, pH, Salt and Temperature?
Have you had whitespot on any other fish?
How long has your tank been setup and how much live rock, and what kind of equipment do you use on the tank?
Ben