That brown algae is a tough nut to crack. All it needs to grow is standard lighting, nitrates, and phosphate. Your system uses up phosphates quickly enough. However, the water changes needed to reduce nitrates also adds new phosphates into the tank.
The best way to combat brown algae is to introduce a few otocinclus catfish to the system. These diminutive catfish graze primarily on simple algaes, hair algae, and brown algae. They won't harm the fish in your pic.
If otos aren't possible, a phosphate sponge in your filter will remove part of the equation that the algae needs to sustain itself. Also, reducing the time the lights are on and reducing the amount of food entering the tank will curtail the growth of the algae.