Ongoing Algae Problem

I am going through an algae issue. At first I thought it was the uglies but it hasn't cleared up. I got a CUC and that made a huge difference but the algae on the bottom of my rocks just won't go away. The top of the rocks are algae free. I am running a skimmer and GFO but the stuff is persistent. The tank has been running for about a year. I have 5 fish, two zoas, and a torch. I have one Hydra that I recently turned down. I feed frozen food every other day. I can't figure out why the top of the rocks are fine but the bottom is covered with algae. I also have coraline growing on the rocks and overflow.