some mexican turbo snails or a sea hare will moe it down but if you go the sea hare route remember that all they eat is hair algae and once its gone they will starve if not rehomed, also they need to be acclimated slowly.
would consider fghting the algae using gfo in a reactor and waterchanges, that will remove phosphate and in return starve the algae.
__________________
........
|