I'm basically new to fish keeping, so I can't tell you what it is, but it sounds a bit like what hit my community tank. I thought it was ick at first because it initially just looked like salt on their fins. Soon it shifted to fuzz. I'm beginning to think it was Columnaris - but that's just a shot in the dark, really.
For me, it started a week ago today. Even though I separated the "sick" fish at the first sign and treated everyone with aq salt, it didn't help my betta or the white clouds. Aggressive salt treatment helped some of the neons. I tried heat with my betta, as a precaution, but he quickly showed symptoms & died in the same 24 hours. At first, frequent water changes with upper tolerance for aq salt did seem to help the white clouds & neons. Throughout the week, the white clouds died. They never showed symptoms. Out of 10 fish, 2 neon tetras are still alive.
I've been super cautious with my other tanks (my hands are seriously dry from washing them so often in water that's just shy of burning me). So far, this is my only sick tank.
Sorry, I can't be more help. I hope someone can give you a solid answer. And best of luck to you & your fish.