This is usually a sign of ich. The parasites basically drill into the skin of the host fish. The fish will then become agitated and rub against rocks and sand. Did you just add your tang to your tank?
His immune system is a bit weak right now from the stress of being caught, bagged, and placed in a new home. In my experience most tangs will inevitable get a slight case of ick when first introduced. Just keep and eye on it and do your best to keep stressors to a minimume, keep good water quality, and keep it well fed so that it can restrengthen it's immune system. As stated above also, soaking food in garlic helps also. Good luck!