That appears to be Diatoms. Dinoflagellates are stringy and will normally be all over the glass as well as everything else in the tank. Vacuuming the substrate will get rid of most of what you can see but they may come back Trochus snails and Cerith snails can both help fight Diatoms. Anything that stirs up the sand bed such as sand sifting gobies and starfish will also help.