It looks like the start of a bacterial infection. I lost three rainbows to bacteria (white fuzzy stuff) it killed them in a couple days tops. One after another. Once the meds came I was only able to save one. I’d treat before it gets worse.
I decided to go with a light dose of melafix on the tank. I took the fish out and am treating it also. Melafix can't hurt even if nothing is wrong so i said why not.Why are you so against treating your tank for a bacterial infection? The female does look like it could have a bacterial issue, but it's not so bad that I would cull the fish. And culling the fish does nothing for the other fish if this is a different disease, they would have already been exposed. Melafix is natural and easy on fish, and all you would have to do is take the charcoal out of your filter (if you use it) and dose for a week.
good choice, I don't like to medicate unnecessarily either but Melafix is so useful and benign that I use it any time I see a fish with an injuryI decided to go with a light dose of melafix on the tank. I took the fish out and am treating it also. Melafix can't hurt even if nothing is wrong so i said why not.