if your tank is fairly new chances are it's diatoms, that's a normal part of cycling. IT's ugly and will spead like crazy but will eventually clear up on it's own. I have that myself at the moment. It's not harmful.
you can try leaving your lights on for only 6-8 hours a day (that made the biggest differece for me)
Make sure you are not over feeding.
10% water changes once a week.
It is best to just leave it alone because your tank will start to procude the bacteria needed to get rid of it. If you use chemicals to rid your tank of this it will most likely come back because you rid the tank of the algae before it has time to grow bacteria to compensate for that. I don't know if I explained that very well. I mean if you use chemicals it will probably come back because your tank never got a chance to produce that stuff needed to keep a good balance, to keep it from coming back.
That's what I've been told and what I have read about it.
Besides I used the chemi-clean for 2 weeks and it didn't help.
Leaving the lights off helped the most.
Good lighting doesn't MAKE it grow but it helps it spread once it's started.