Perhaps you could chat with TheOceansTraveler regarding her tank. She has a similar setup as you but on a bigger scale and her tank looks fantastic!
I'm with KimP, my urchin is an indiscriminate mower of algae on rocks so I'm surprised it didn't just mow over the GHA. If you could, I'd manually pull what I could and then remove the rock from the tank, drip/spray some peroxide directly on the rock, being careful to avoid your anemone, and let it bubble a bit to kill off the algae. If you really wanted to be careful with the anemone, see if you could submerge the rock but leave the side with the anemone in the water and the rest of the rock exposed to the air and hit it with peroxide then.