Yep, looks like hair algea. The most common reason is trAtes and/or phosphates (which can register low to none b/c the algea is consuming it). Are you using tap water? That will fuel hair algea. There are other factors like lights, lighting time, clean up crew, amount of fish, etc. Critters that can help (but the source still needs to be found) are lawnmower blennys or emerald crabs (the last from hearsay). Test your water for the above and add what your lights are and lighting time. Well, basically any other info you can give would help.