Probably diatoms. Many new tanks develop this condition. It is from excessive silicates/phosphates in the water. this could be due to overfeeding and/or your tap water containing high levels. It is easily rubbed off and poses no problems other than being unsightly. If you have room in the tank you could add an oto catfish and it will be gone within a couple days. Just remember if you do this to feed him some algae wafers after he's done cleaning. Alternatively, there are filters that remove excessive silicates/phosphates from your water. In a bigger tank bn plecos are wonders at removing the stuff.
