Can you elaborate on the "brown algae" ?
Is it sort of dusty/powdery? If so it's diatoms, fueled by silicate. Not a big deal, just unsightly. If your using good quality RODI now, it should pass with time. Also if your dosing iodine, too much can contribute to diatom growth.
Most phosphate reomvers (rowphos and similar products) will remove silicate along with phospate. Use with some caution though, in some cases these phosphate removers can mess with alk levels if used too much/too fast IME.