Color loss and/or bacterial infection most likely, could even be a simple abrassion. All easily fixed for the most part. First you need to get some SW
mixing for water changes. Mix well with a powerhead and heater for 24 hrs. You will need to do about 3x20% water changes over the next week. This will reduce any DOC
in the water that might be feeding potentially harmful bacteria. You need to keep up with at least weekly water changes after that to keep it in check. Always used well aged/aerated SW
though. What are your water parameters?
Next step is feeding. What does the tang and the rest of the tank get on a regular basis?