I battled this stuff in a tank a long time ago for months. I think it must have been something leaching out of whatever substrate I used, but I was vacuuming this crap up every day, cutting way back on feeding, etc. I think eventually I ended up breaking down the tank, cleaning the whole thing and replacing the substrate, which was pretty easy on a 10gal.
As others have said, cut back on food, cut back on your photoperiod, and do lots of water changes and vacuum out as much of that stuff as you can get every time, and eventually you should beat it.
lol 2 tetras and 5 shrimp in a heavily planted 20 G? Test your water, it could easily be lack of nitrates.
No big deal though. Provide proper conditions for the plants, and increase flow and the plants will outcompete it.