Looks like a gram positive infection which is slower to act and develops with an injury or poor water quality. The top ahead appears to have begun with a minot injury and treatment will be mnecessary using Seachem kanaplex and water quality maintained along with added aeration using an air stone. It has not yet become sore which is good.
Is fish eating?
Breathing normal or labored?
From what I can tell, this is a quarantine tank. After use f Kanaplex, follow with PraziPro applying initial dose at 85% of recommended and apply initial dosage known as an interval for 8 days, do a water change and do one more 8 day dose interval
Use airstone with prazi as it does reduce both oxygen and appetite. If running a skimmer, leave cup off first 24 hrs as skimmer will go nuts from the glycol solution in Prazi