Is algae growning in your tank?, it can be from that or else to much co2 inside your tank. I got that once, I end up doing a 75% water change and turn off my lighting for 3 days straight and it all went away. But it seem like dirty waste scum due to less water changing.
I always get brown stuff on my plants. When it is on hornwort & anacharis I just throw away the worst part and keep the new growth. When it is on anubias I rub it off the leaves with my fingers. so far it hasn't affected my duckweed or amazon sword.
I figured it was due to either hard water or high ph. it's like rust.