is it a newer tank and are you seeing some of the brown on your decorations? If thats the case then they are most likely diatoms and will wither away as the tank becomes more established and if they brush off easy then that is also an indication of diatoms. Be sure to not throw that messy piece of filter media away but instead rinse it out in a bucket of tank water to keep your cycle going.
I rinsed out the filter media today in a bucket of old tank water. It made the old tank water brown and cloudy. There is sometimes some light brown stuff growing on the glass but it doesn’t exactly grow in spots.
I had the tank for over a year with a betta. Awhile after she passed away, I added three platies. This was about four months ago.