Much luck man. Sadly my 20 long is afflicted with this stuff too and its just nasty.
Already done a complete tear down to clean/dip plants individually in H202 and Excel (albeit a rather quick dip for most things). Stuff just comes right back. I have toninas and pogostemons in the tank as well so I'd rather not nuke it with chemicals if possible. Going to get it cleaned up as much as possible this evening and try the blackout method then run lean on ferts and drop the lighting to like 2-3 hours a day after that before I give up.
Just kills me that everything is growing so well, dosing is on target, CO2 is stable @ ~30ppm but I can't shake it. I had it before the tear down but I more than likely made the problem worse by adding some fresh aquasoil back in during the re-scape.