Sea Hares are only quick fixes. They will eat all the algae, poop it out, and it will be left to decompose to fuel more algae. To get rid of algae, you have to kill it at its source by depriving it of at least one essential element of life. Light, Nitrate, and Phosphates are the most controllable elements of life for algae. If you can accomplish not having any of either of these, the algae will die. Water changes and nutrient export coupled with better tank husbandry, moderate to high flow to keep debris off of rocks, and lowered feedings over time (sometimes months) is the end all success with the battle against algae.