Looks like brown algae to me, I have/had it to in annoying amounts. Ive read that its caused primarily by silicates leeching from things like sand/glass, which is why you only normally find it in new setups, but I dont know how true this is.

One things for sure though, my Oto's LOVE it! Get some as Tom says, sort out your CO2 and all will be well. Those 2 things have knocked mine on the head no problems at all. Oto's really do rule the world. I love them.