If it is a brown/rust colored dusting, it is probably diatom algae. Diatoms need silicate to create their cell walls, silicate is typically what fuels diatom growth. You can either use a silicate removing media, or you can use herbivorous snails to clean it up. I use snails, I also wipe the glass 1-2 times per week, when the diatoms are scraped from the glass, the corals get a phyto treat. Another big algae fuel is phosphate. Phosphate is easily removed from your tapwater with an RO/DI filter.