Check the phosphate and nitrate levels with a different test kit (some can be notoriously inaccurate, from personal experience), and depending on the algae type, try to add the relevant clean up crew, as well as reducing light levels if needed. I've just won a 2 month battle with some serious bryopsis or hair algae, and I only beat it with reducing the lighting to 6 hours a day, rock scrubbing, a borrowed lettuce nudibranch who did a sterling job and adding faster growing macroalgae to soak up whatever nutrients were left after twice weekly water changes. Hope this helps and good luck!