My guess is either a severe diatom bloom or dust algae. Dust algae starts out growing on the glass, and eventually falls off into piles. If your glass is turning brown, it's probably diatoms. If your glass is turning more of a green color, it's dust algae.
The good news is both of these algae types should eventually clear up all on their own. BUT, you have a heavy fish load, which definitely doesn't help. At the very least it will help this stuff persist much longer than it otherwise would. Maybe increase your water changes to twice a week if only to keep up on vacuuming that stuff out before it can build up. Your clay substrate could also be contributing to your problem. At 5 months old there is a chance it is still releasing some ammonia. The solution might be a lighter stock, and more plant mass to make use of the nutrients provided by the substrate.
