Help getting rid of brown algae

[Roblox84]

I have had this brown algae growing in my tank for a few months now and I can't totally get rid of it. It syphons up very easily but it grows back the next day. I have good circulation and nitrates and phosphates are undetectable. I also have my lights on for 6 hours a day and i only feed fish once a day. I do water changes every 2 weeks. You can see a video of this algae here:
http://www.youtube.com/watch?v=gJYW_e91mjo

 

[Uncle Salty 05]

Not sure, but it could be dinoflagellates.
I don't think diatoms or algae would grow back that quickly.