Is green algae just a fact of life?

I have a 14g biocube with about 14lb of live rock. I bought the expensive stuff thats already purple. In the tank I have two skunk clowns and one bubble anemone. Also a couple snails and crabs for cleanup. I have filter floss in one chamber which i replace every few days. I have chempure elite in another chamber and lastly I have a uv sterilizer that I run for an hour a day. My water temp is about 80 degrees and my nitrite, ammonia and PH all read fine. My nitrates tend to range around 40, but the filter floss I'm using has a nitrate reducing chemical added to it so that has brought that number down to about 20. I'm using the stock lighting that came with it which runs from 11:30 - 7pm. 20% water changes every two weeks. And lastly I feed my fish frozen brine once every other day. Any thoughts on what could be causing the green algae growth or is it just a fact of life that I'm gonna have to get in there and scrub the glass. I also notice it on any of the white parts of the live rock which are now green. Thanks for any help and tips.