Yes - the coloration on the head is called "epithelial thinning". It is pretty common in hepatus tangs,. It sometimes develops into classic head and lateral line erosion, but not always. Lots of speculation as to what causes this in blue tangs, but stress is one factor, diet probably another. for HLLE, the one known factor is carbon use.
So - the cap on the plaque seems to have fallen off? That is very interesting. I was I had that fish in hand as a skin scrape would be interesting. I doubt that it is from an injury or "heater burn". I'm leaning towards bacterial, as fungal lesions are very rare in marine fish.
Since it is in your DT, sounds like your treatment is the only course of action - although I don't hold much (any) stock in reef safe medications, since the pathogens they purport to eliminate are sturdier than corals - how can they selective kill them without harming coral? At best, they are considered tonics. What happens is: the fish dies and the aquarist shrugs and thinks, "oh, I must not have caught it in time". Or the fish lives and the aquarist thinks, "great it worked!". Trouble is, some localized bacterial issues with fish resolve on their own, so the fish could have recovered without the "reef safe" medication.
Jay
www.reef2reef.com
