the main cause of almost all algae is low levels of co2. you dont have to go all out and build or buy a co2 system, this could be as simple as dosing with Flourish Excel as this is a source of carbon for the plants.
im pretty sure youll see the algae go away if you start adding some carbon in there with Excel.
the thing is, that as long as you dont address the underlying issue (lack of co2), the algae is always gonna come back eventually