I've read through this thread and wanted to throw in some of my experiences to help clarify some things.
#1- I have imported fish from all over the world and never heard of a powder grey tang.
#2- Tangs and surgeons that grow streamers are usually only the male fish. Females don't have the streamers so the fact that they are missing is meaningless.
#3- It does look more like a "Cat/ Gold Rim/ White cheek Tang. ( All three names are the same fish, just from different areas of the world) The fact that it is suffering from a disease now means that it may not be healthy enough to be showing all it's best colors. I had imported a rare WHITE Yellow Tang from my Hawaiian supplier. It's exoticness was really something to see. It really stuck out against the school of other Yellow Tangs until.... We always fed our Tangs ORGANIC Romain lettuce from a local grower. After a week of eating healthy food, that white tang turned as yellow as the rest of them. Obviously, this fish was suffering from a nutritional deficiency and may have just been a holdover fish in my supplier's supplier's tank. Rare, apparently not, Sick, possibly
Something to think about
#4- Fish stores are known to "make up" names to describe a fish that may have been hard to sell or even get or just a bit slightly off of the "normal" ones. Back in the 1970s, I used to sell an Atlantic Clown Trigger. I dare anyone to find an Atlantic Clown Trigger that is not a transplant from the Pacific
. What I was selling was a white spotted filefish. In the 1990s, I was working in a wholesale fish house in Miami, FL and a new diver/ collector brought in some, what he called " Atlantic clown triggers." 20 of them to be exact. I went to look in his bucket and don't you know, they were juv. white spotted filefish. So that name had stuck for almost 20 years. lol So, if you can't find a fish in all the many books that are out there on fish, chances are, either they are a new discovery and would be a fortune to buy, or they are not a real fish
Hopefully, once you cure this fish of any disease it may have and get it on a good diet, all these questions will be answered. ( BTW, if the white doesn't come out on his face, it's probably a gold rim/ achilles hybrid. We used to get them from Kona all the time.
Hope this helps