I agree with Johnnyxxl ....... Unless, by some strange coincidence, you're having the same or similar problem as I am/was.
My plants and some of my substrate were just covered in brown stuff (last few months). I knew it wasn't diatomes because it wasn't a slippery biofilm. Upon real, real close inspection, I realized it was little individual, brown crumbs.
I thought "WHAT, MY FISH CANNOT POOP THAT MUCH?!!!" I have 7 nanos and a 2 inch, full grown pleco in a 20 high with good filtration. 😳 And, I never had this problem before now.
So after more sleuthing, I realize that the crumbs were thickest in areas near a piece of Malaysian driftwood that is 2 years old. Pulled it out and could tell it was getting soft and breaking down, threw it away. Vacuumed the gravel and plants and most of it is gone, not all. Now, I monitor my tank for a month or two and see if I fixed the problem.
I realize that a small part of my brown crumbs may have indeed been fish waste, so I know that won't change, but I cut the food back a bit. I make sure I pull out dying leaves, maybe leave a couple for my shrimp. I wish I had better vision; might have resolved this earlier.
Just something to ponder. What's in YOUR tank?