Looks like I am seeing two types of algae growing in there. Caulerpa and something else red. Which is the OP referring to? If it is the green, you can just work on pulling it and replace with something like chaeto and then keep removing any that grows back. If it is the red, then I guess what James M said and scrape it and siphon it out. Probably some other way to control it too, but I was a novice last time I had tanks (and had moderate algae everywhere) and am only now the last year or so been prepping to get back in so don't have first hand experience with it. Just wanted to comment since people seem to be looking at two different things in the photo.