This sounds like a case of diatoms and there are a few ways to deal with them. While they are very common, you can control them to some extent by reducing the amount of time that the lights in your tank are kept on as well as reducing the nutrients that diatoms would feed upon.
However, many people find that it is easier to add a small group of
otocinclus catfish which remain small (one to two inches) and would fit really nicely into your current tank set up in my opinion. Many people love these fish for quite a few reasons and from my experience with keeping them, they do a really great job on a lot of types of algae.
