Could be a number of things.
RO water is good. RO/DI is better because RO alone will not remove certain nutrients that plants like.
Probably greater likelyhoods are:
1) Overfeeding. Turn off the external filtration (i.e. ok to leave Koralias and the like on) while feeding and feed no more than what can be consumed in a minute.
2) Light cycle. 8hrs is generally plenty of light (give or take depending on what you're keeping.)
What are water params (Nitrate and Phosphate esp.)? Also pic of the algae would be helpful. If it's corraline, that's good. If it's hair, not so much.