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Small tanks are a challenge to get a handle on the water chemistry. It's easy to feed too much and the dissolved food and unstable water properties make for a great environment for all types of algae.
I'd slowly reduce the amount you feed the fish and plants. These small, living things get very little food in their natural living places, and can't begin to use all the food we tend to give them. So, there's extra in the water for the algae.
Keep the water clean. Do weekly water changes to remove the nutrients in the water and feed just a little. I feed my fish and plants a couple times a week tops. That way, the fish spend most of their time looking for leftovers. This keeps my tanks very clean and free of visible algae.