You can lower the amount of lighting during the day. Also, if you have an hour inbetween the day when the lights go out, it sort of "stunts" the growth. It has to "regroup" and start regrowing. I had this in one of mine, and this is what I did. That was 3 weeks ago, I did a thorough scrubbing and massive filtering, and now it's virtually gone. My light goes out for one hour around lunchtime. Also, since you don't have LR
go ahead and flip your rock over (so the algae growing isn't lit up) and this will help that to die off. Lots of Luck!