Try putting your rock in an outdoor tank outside in direct sunlight. The rocks I use for my RES turtles get that kind of algae growth and I get slightly green water but no algae on the glass. I do change the water every 5-7 days though and spray the side of the tank with a hose to clean it off.
Thanks all! The algea does grow everywhere else also I just keep it scrubbed lol. It's not normally that green but my old lady left the lights on for a week when I was away because my old fish needed them on all the time. She don't know better ha