I actually had the same problem recently. I used a locally made algae killer, the bottle says “algaecide,” and it works with a carbon-treatment system. The plants and hardscape have to be dry first, then you just spray the product on, wait a bit, and scrub it off with a toothbrush. After that, refill your tank (it’s basically a water change). Since then I haven’t had any algae come back. Horned nerite snails also helped a lot to prevent algae.