The algae on the glass behind the heater is probably green dust algae. If you remove it by wiping it off the glass, with the water level below the level of the algae you can go a long way towards eliminating it. When you scrape the glass, letting the scraped off algae stay in the water, the algae becomes free swimming again, and just colonizes larger areas of the glass (as well as the hardscape and some plants). Or, Plantbrain is recommending bristle nose Plecostomus catfish as a faster, more certain way to get rid of it. They eat all of the GDA they can get to. But, there are some downsides to those fish, too.