What you are seeing is algae, brown hair algae or black beard algae.
This kind of algae is not so dependant on light and so is very hard to get rid of. I have had success with it in the past by changing 10% of the water every two days, and test your water for phosphates if you can. Also test your tap water, some water sources are high in phosphates before you even add it to the tank so algae can thrive. Phosphates are a product of the waste produced by fish, fish food and plants decaying so tank maintenance and water hygiene are the place to start.
I have used phosphate absorbing resins with good results in the past, API Phoszorb being one I have used again and again. With this kind of algae, fish that would usually eat lower form plants can be unreliable. I had a Gibbiceps Plec that loved the stuff but Siamese Algae eaters are the only fish that eat it as a preference (Thanks CFC!).
Ken