That involves a pretty large amount of skin, and if it is also on the other side, that makes the issue even worse. Fish need healthy skin in order to maintain proper osmotic balance with the water. When the skin infection gets to a certain level, they cannot do that properly. Lowering the specific gravity down to 1.018 helps them balance, but of course, you cannot do that with invertebrates present.
There is not any true antibiotic that can be safely used in your reef tank at a level needed to stop an infection like this. If it is gradually getting worse, I think the only option would be a treatment tank. Some people have tried giving high dose antibiotic baths in a bucket, with a heater and air stone for three hours a day. That is a poor second choice of treatment, and don't try that unless you are confident that you can catch the fish up daily without causing more damage.
Jay