It's hard to tell from the picture. Looks like maybe Cladophora. Manually remove as much as possible. I'm in the process of trying kill a small amount that somehow made it into my tank. I'm using spot treatments with H2O2 using a syringe. Make sure you turn off any filters and powerheads while you do the treatment. It has seemed to help to an extent. It might take repeated treatments. I'm going to try spot treating either daily or every other day for maybe a week and see how that works. It's supposed to be one the most difficult algae to eradicate, as it thrives in the same conditions that aquatic plants do. There might be some fish that will eat it, but that's not exactly practical for me to get more fish just to get rid of it, especially when they're fish that I don't want in my aquarium. I hope this helps. I have yet to find a definitive treatment to kill it. I'm just doing the best I can.