It appears that the outer mucous coating has been destroyed. The result is an immediate, significant bacterial infection. Since most bacterial infections are Gram Negative, you want a Gram Negative antibiotic, like Maracyn Two. I would choose Maracyn Two for saltwater fishes because is goes inside the fish as well as treating the surface. Move the fish to a hospital tank, setup like a quarantine tank. If you are going to use Maracyn Two for Saltwater fishes (or freshwater fishes) start off with a double dose. Double the initial dose recommended on the package.
You must also help the fish with its coating. While treating the fish with the above antibiotic, also treat the fish with either of these two products: Pro Tech Coat Marine or StressGuard.
I would move very quickly with the treatment. I hope the fish doesn't get any kind of fungal attack (but this would be 'normal'). I would recommend you obtain some Betadiene and have it on hand.