A picture would be great.
1. Ammonia could be an issue - how old is the tank, etc - have you checked it. (PS - ammonia in the shipping bag can also be an issue - how did you acclimate). Do the gills look red?
2. Could be a bacterial infection - and a hospital tank/antibiotic treatment might be in order - is it staying the same, getting better or worse?
3. It could be the age. I have seen this on younger tangs - where the face 'looks' red. But without a picture impossible to say. I'm assuming this is a young tang.
4. Nitrates would not be an issue - in fact 'low nitrates' may suggest an ammonia problem.
The good news is that he is eating/swimming, etc - Hope this helps!