Setup up a siphon hose with a toothbrush attached to the end of the siphon. Scrub very slowly. The idea is to let the siphon suck out as much algae as possible. Best if you set the siphon to return to the tank with a filter sock on the other end of the hose. This way you can work without interruption until as much of the HA is gone. Work s-l-o-w-l-y