Hello Feline,
Looks to me like you have small case of brown algae. Brown algae is actually seen less frequently in aquariums for it flourishes in low light environments. A small amount of algae is normal in tanks, and is actually quite beneficial, as it reduces nitrate. But if your like me, can't stand the sight of it. However, if algae runs rampant in your tank, this usually indicates an inbalance and poor water quality (usually high nitrate, and phosphate). It makes a wonderful warning sign.
My suggestion would be to try increasing your light. Check your temperature as well, depending on the type of fish (I'm assuming you know the minimum temperature requirements) lower your temperature to below 78 F gradually over the next few days. I would try to get it down to around 76 F but then again, depending on the fish. Also make sure you are not overfeeding, as this raises the organic load, therefore increases nitrate, therefore providing more nutrition for the algae.
And please resist the urge to try a commercial algicide. Trust me. Although it may provide you a quick easy fix, it does not solve the problem. Once you stop dumping in the chemicals, it will come back, and worse. And continuous exposure to the chemicals can be detrimental to your fishes health.
Mogo