It looks like biofilm. It occasionally happens on plastic materials, but more common on organic materials.
Same time, there is some diatom algae mixed in as well, as seen as the fine coating of brown dust on it.
Being in the filter flow, it likely traps a lot of organic material which would speed up some growth of algae, biofilm, and diatoms.
How new is the tank setup? How often do you water change? How long are the lights on?