I had the same issue before....it comes and goes. when you grab the puffy pooh balls, do they seem foamy? I had that slime grow on my waterfall rocks and I would have to gently pull it off....I hated that stuff. :(

I think it's kind of like a blanket weed, but brownish in color...I'm pretty sure I know what you're talking about. After a couple years, mine went away.

I think it's either something different from what you have or manifesting itself differently. This stuff forms discrete puffy clumps an inch to 4 inches in diameter.

Since it's puffy in the water but collapses into slime as soon as I take it out or disturb it, I suspect it has something to do with gas (methane?) forming on the bottom, then lifting the yummy nitrogen-rich sediment to the top. I dump it on pond-side plants; they don't object.

My guess is that it is DOCs (dissolved organic compounds) and you may need to do water changes more frequently.