If were to bet, I would bet on your tank being very new (couple months old?). And this is most likely diatoms. Increased flow will help keep your sand looking clean. Depending on the size of your tank, you could get a fighting conch snail. Very cool critters to watch, and do an excellent job keeping the sand white. You could do one if you had a mostly open sand bed at around 20 gallon tank size, or 40 if most of your tank is covered with rock.
This is all assuming I am right on the diatoms. More info is needed to confirm or deny.
EDIT: Your last post changes things although it still sounds like diatoms and a conch will still help if you don't have anything to stir your sand bed. You might also consider changing your sand. Siphon out about 1/4 to 1/3 of your sand bed and replace at a time with at least a few weeks in between. You can prewash the sand, and use a section of 2" or larger pvc with a funnel to get the sand in the spots you want without making the tank extremely cloudy in the process.