I've dealt with this gunk on one of my crowntails fins for 3 months now. Finally, about 2 weeks ago, I discovered the cause. Evidently, leaving the filter on overnight causes it. Completely serious. I've been playing around, sometimes having it on overnight, sometimes having it off... and I only see it if the filter was on.
This particular fish really hates the filter. Whenever it's on, he hides under an ornament in his tank and doesn't move, he rarely even comes up for air. The filter is baffled and hardly moves the water's surface; and it's the same filter I use for my two other tanks without any issues.
I guess this stuff can only attack him if he's sitting still. But my tanks are all very clean, so I'm still questioning what exactly this stuff is.
But what can I do? If I turn the filter off at night, the heater doesn't work well; the bottom of the tank will be 75 degrees in the morning, and the top will be 80. That can't be good for him.
How else can I circulate the heat around the tank?