Are you using yeast generated CO2? I saw something similar in an aquarium when I experimented with bubbling air through the yeast reactor. Some of the yeast (presumably) made it’s way into the aquarium and grew into a pale blond filamentous slime. My scaleless fish became very agitated, flashing on the rocks and wood. Bad stuff.
I had this also when my tank was new (less than 2 months). It was brownish and almost like a tumbleweed appearence,easily removed by twirling a toothbrush or filter brush . I manually removed daily, upped my co2 and it disappeared within 10 days never to be seen again.