So sorry to see this beautiful tang in this condition. I'm sure you, dochoot, are stressed over this.
I'd like to know more background on this fish. You say this condition just developed on its own? What other fishes do you have in this aquarium? What inverts? What corals? Let's look at that long list.
This looks like a severe injury.
The fish needs special attention. I'd recommend moving it into a hospital tank. (Not an easy task I'm sure. But I've seen 'professional' aquarists use two nets and catch a fish in a reef tank in under a minute.) Once in the hospital tank, the treatment begins.
It's always difficult to 'guess' what kind of bacteria will attack such a wound and condition. If you have a microscope or accesss to one, it might be worth trying to narrow down the type of bacteria that will undoubtedly set in.
Gram negative bacteria are the usual culprits to take advantage of an injury, so I would recommend you get or have
Mardel Maracyn® 2 and use it as soon as you can. As far as Gram positive bacteria. . .Metronidazole (controls common bacterial infections in fish. This exerts a bactericidal action on gram positive and some gram negative bacteria) works best when dosed properly in food (at 0.50% by weight). Next you want to be prepared for a possible fungal attack. A fish in this condition is a fungal target. So get, but don't yet use, Pimafix.
The next thing is nutrition and food supplements. Get one of these:
Vita-Chem Marine
Zoe
S.E.L.C.O. Boost
Add the vitamin to one feeding each day. Next the fish needs a boost in its own immune system. And for that. . .
A sick or healing fish should also have beta glucan included in its feed until it is healed. Beta glucan is the same beta glucan humans use and can be bought from a health food store. The fish needs to eat about 20mg of this every day. Beta glucan helps the fish restore its natural immune response to some diseases and conditions.
Last but not least are the foods. This is a herbivore. Feed seaweed/algae every day, even twice a day is good. Nori is okay too, but not the kind that has been roasted/heated. Look for the sun or air-dried kind. Also provide freeze-dried foods to soak up the supplements. Read this post:
Fish Nutrition and please take this fish's nutrition very seriously.
I'm sorry if some (or all) of the above is what you know. But, I don't know what you know or what's in your fish medicine cabinet.
At your next opportunity please post a 1 minute or longer video of this fish. I want to see both sides, how it is breathing, and swimming. Thanks.