It could be an injury. Tangs make occasional 'mad dashes' across the aquarium. Not being the sharpest fish in the sea, they do run into things.
I notice a slight fading of color though which has implications of not the best nutrition. The foods you list don't contain enough vegetable matter. Out of 21 feedings, no more than 7 should be of brine shrimp or mysis. DO NOT FEED straight brine shrimp. Always feed gut loaded brine shrimp. Feed alternately spirulina gut loaded brine shrimp and fat loaded (HUFA or Omega 3) b.s. and just once or twice out of that 7, mysis. The other feedings need to be foods high in greens, PLUS the daily addition of algae. Offer not only nori, but alternate with algae and if possible algae of different colors (green, brown, purple, red). Other foods to consider are foods containing kelp and spirulina, Formula Two, broccoli flowers, etc.
Soak one feeding in vitamins every other day; soak a feeding in fats every other day; while it is healing put some beta glucan into one feed per day. After healed, stop the beta glucan but continue the vitamin and fats soakings.
Keep an eye on the injury. It may worsen or get infected. You should be prepared to treat for an infection. I recommend getting a hospital tank up and running and obtain some antibacterial medication such as a good antibiotic.
I think if you immediately improve its diet, make sure your water quality is the best you can have, and that you are up on your maintenance and detritus removal, the fish should heal AND return your fish to its normal brillliant colors.
Consider using a moon light at night, if you aren't already.