If your using RO/DI water I would suggest stepping up the water changes to 15% weekly for the next few months. This may sound extreme but when it comes to nuisance algae everytime you remove or trim it back you cause small die-off which in turn will add pollutants back into the system. After a few months of weekly water changes and the other suggestions above, you should start to see a big improvement. Alkilinity should also be checked as if it's low, it too can also help contribute to algae problems.
Starving the system of nutrients by doing these weekly changes should eventually have a payoff, plus it will help replenish desired levels of Ca, Ph, and Alk.
Goodluck and patience is a major factor
