It looks like an injury of some sort, but in a healthy fish I would think that would heal up on it's own. I don't think medication treatment would really be needed, just time, observation, and some more frequent water changes. Medication should only be used when it is definitely needed and there are no other options. At least, that's my opinion. Some fish are more sensitive than others to the negative effects of medications, and I believe that Tangs / Surgeonfish fall into this category, so you have to be careful. You could do more harm than good.
If the fish is still eating, that is a good sign. I would just wait it out and see what happens.
However, since you have already started medication, I do not know what to do. Since it is expired medication, it may not be doing anything. I'm not familiar with the brand of medication, so it's difficult to say what the proper method of removal is, but water changes will definitely do the trick. Activated carbon could also work, but may not depending on the type of medication.
My tanks: 120 Reef w/L2 Algae Scrubber, 60 Reef Pond w/floating Algae Scrubber, 40 Breeder Reef w/L2 UAS Tester Algae Scrubber
I maintain: 144 SW Reef w/L2 Algae Scrubber | 200 SW Reef w/L4 Algae Scrubber
Special knowledge: Algae Scrubbers, Acrylic Fabrication