It could be the start of HLLE. There can be different reasons for this disease to show up. Poor water quality, lack of proper food, the use of carbon.
If your readings are high for ammonia, nitrite or nitrates, then a water change is in order, and parameters need to be monitered until these readings are normal. Especially in a QT tank. Has this tank been cycled? You might need to do even daily water changes if it hasn't.
You said that the fish is not eating yet. Have you tried one of the frozen (thaw first) mixes? Live brine shrimp? Don't overfeed especially if the tank isn't cycled.
Carbon has been linked to stripping the slime coat off of angels and tangs. If you are using this, you might want to stop for awhile.
HTH
P.S. Does anyone know why the disease forum is closed?