Fungus in a SW tank is a very unusual thing to see due to, well, the salt content in the tank.
It looks more to me like a wound. To treat a wound the best thing to do is insure you have outstanding water quality. If you run carbon, you might consider changing it and I would suggest a large water change and then another one a couple of days later.
I would do nothing to stress the fish further as in chasing it down and moving it to a hospital tank or adding Melafix or Primafix.
Pristine water and a good varied diet (multiple times daily) are in the fishes best interest right now. You need to watch for a bacterial infection at the site (redness or the area grows in size), if that happens you would have to remove the fish to try & treat it but hopefully you won't get to that point and since the fish is well established in your tank, food & excellent water quality are your best options right now.
As far as the black spots - that could be due to the frogspawn hosting. Contrary to popular belief, sensitivity to stings in a fish tank increase as time goes by, they do not necessarily decrease. Since you mention that
the black spots showed up after the other issue, I'd venture a guess that the fish is being overly sensitive due to the fuzzy spot issue.