Those sound like diatoms -- a type of algae. Diatoms are totally normal in a new tank. You'll get diatom outbreaks pretty regularly for the first 6 months or so. As your tank matures, they will appear less frequently until they go away altogether. There isn't anything you can do about it except wait it out. It also helps to never use tap water (use only RODI water), and not overfeed your fish. Like other algaes, the diatoms will thrive on the nutrients found in tap water and produced by overfeeding.
How low is your pH?