Nothing prepared me, as I was going to bed last night, to come across something like this.
This didn't come from filter debris or anything I can imagine.
I've done tanks over the last 20 years, mostly salt, and now now mostly fresh water, and I've never seen anything like this. I googled some kind of water mushroom and that only pulls salt water photos.
The kuhli loaches were enthralled with it, eating it and zipping through it, which released the inner brown debris. I fished it out and it was like a fungi. When it dried, there was hardly anything left, just bright green crust.