I'd get 2 otocinclus to eat the algae...... I don't know what causes it. Maybe the algae is a similar colour to the banana plant leaves so you don't notice it? Older leaves also get algae, that have been in the tank water longer to have algae settle and grow on the leaves... Maybe shorten the length of time the lights are on.
I don't fuss much about algae, it is natural, and as long as I can see the fish and it isn't blanketed on plants, I don't mind it being around.