You can try 2 things here-you can try treating her with an antifungal med for a course of treatment and see what happens. If it clears up, then you know it was fungus. If it doesnt clear up or gets worse, then you are most likely dealing with columnaris which is a bacterial infection and antibiotics will be necessary. Which it is, im not 100% positive (im not a vet) but fungus & columnaris will eventually kill your fish. I would also dose with aquarium salt to lessen any fluid loss & increase her mucous coat.