I've seen a few posts that describe this stuff but no one seems to have a good response. I have 90Gal. high-tech tank w/canister filter, e.i. dosing, 50% WWC, and a large cichlid that creates lots of wastes. The alga appears on older leaves and is light brownish to nearly black in color. It looks almost like very fine dirt or debris but does not rub-off the leaves and seems to grow from out to inner leaf. When I remove affected leaves and scrap some off, it has a brown/green appearance. I recently treated my tank w/e-mycin (for BGA) but it didn’t affect this stuff. I am currently trying excel, higher C02, less light. Can anyone identify this stuff??? Or provide advice??