sorry it's not brown, it's green algae,,,,,

[muck]

I see you are running a fluval 404 on your tank.
With or without filters? (sponges in particular)
How often do you clean the sponges if you are using them. Sponges can trap a lot of gunk and if not rinsed often and good enough might be contributing to your algae farm.

How is your skimmer performing? Is it pulling out a ton of dark green/black gunk?

Also how big is your cleanup crew?