It's diatoms which are common in new tanks. Commonly they are caused by excess silicates. Diatoms will eventually go away or you can get get a couple snails (nerite, ramshorns, mystery) or depending on the size of you tank, which you didn't say, you could get a bristlenose pleco (personally I wouldn't keep a BN Pleco or any pleco in a tank smaller than 30 gallons-IMO).