What kind of tank is it? You can treat the tank ONLY if it is strictly fish-only. Meaning no live rock, nor corals, inverts, snails, crabs, etc. Also would like to wait and see how this develops in a day or two. See if you can get better pics in a day or 2. Keep an eye out for the fish rubbing itself, shaking, darting about, gilling rapidly.
You'll need to do hyposalinity. Any use of medications will contaminate the display tank for the use of live rock, corals, inverts. Below is a link to the procedure. Strongly suggest that you set up a quarantine tank for future use if you plan to keep fish.