Try as hard as you can to remove that. I allowed my tank to reach that level of algae growth and soon I had hair algae all over the place and my tank was barely visible from the outside. No matter how hard I tried, I couldn't get it under control. Eventually my little cousin tossed a whole bottle (it was one of those fat round bottles the size of 2 or 3 regular ones) of flakes into the tank and my water turned purple (flakes were purple and I wasnt home) and then my tank was out of service (and still is). Anyways, point being I was unable to do anything about until I dismantled the whole tank (lost all my fish/corals/LR
) and cleaned it out. I would recommend that you start cleaning ASAP. My biggest problem was I wasnt consistent and persistent in my cleaning and there were period of time where I didn't keep up. Start by cleaning off the glass and removing any equipment in the tank that has algae on it and thoroughly cleaning it out. "clean-up" crews do help to a certain degree but wont make much of a dent with the sand. Don't stir the sand, it will only get dirty (it will be nearly impossible to clean up) and algae will get stuck underneath in little pockets where they will die and once those pockets are exposed they will be harmful to the tank. Use a siphon hose as others have recommended and start taking it off the surface of the sand and rocks. I can guarantee that you will lose a noticeable amount of sand so make sure you have some to replace it with. If you notice hair algae on any of your rocks, throw them away and get new ones. Hair algae spreads like wildfire and once it infests the tank then you will have a hard time dealing with it. As of right now the tank doesn't look that bad compared to what I had to go through and so from experience I can tell you that it wont be too hard to bring that back to normal, but my advice is get to work ASAP as you dont want to make things worse.