Sadly I’ve ran into a few diseases that have done this, seemingly out of nowhere too. How did you handle it and what did you do to prevent it from killing anything else you added into the tank? I for one bleached the HECK outta my tanks that had this and then kept rinsing them out with vinegar and water.
I believe that the fish major thing that wiped out one of my tanks when I was starting with either Ich or it possibly was something else in the form of a parasite.
As far as cleaning, I used a ton of vinegar, magic erasers, and water to clean it fully out. Then I setup new filter media, and let the tank run through a cycle for a month and half.
By then anything that was there is surely dead without any hosts available.
I think mine was some type of parasite that kills all of my fish in like a week.