I don't want to scare you but I do want to tell you of a very odd disease I dealt with and it started in my bettas. Basically it looked like a patch of white-ish (not fuzzy, very flat and looked like their skin) skin/scales. As it began to spread I noticed the fish were actually paralized in the area of the discoloration. It was very sad to watch and I couldn't find anyone with suggestions on what to do. This was a very fast and hard hitting disease once it got started. After my female betta died (or rather, I euthanized her) it started spreading. I lost a total of 4 fish in 5 days. It was as if the process sped up
I ended up treating the tank with melafix and then another anti-fungal med. However, I also started quarentining fish as soon as I saw any symptoms or signs. The disease never came back and it was gone as quickly as it came. Very odd.
Anyway, the reason why I said all that was just to give you a heads up. You probably (hopefully) don't have the same thing that my fish got. I would QT
your betta and treat with the Jungle Fungus Eliminator (I believe that is what I used). Good luck!