Beard algae
Grows on plant leaves and is bright green. Individual strands have a very fine texture but it grows in thick patches and looks just like a green beard. It grows up to 4 cm. It cannot be removed mechanically. This does not indicate bad water quality but grows very fast and overtakes the tank, making it a "bad" alga. Can be eliminated with Simazine (Aquarium Pharmaceuticals "Algae-Destroyer").
SAE's will eat a little of it, but regular water changes, understocking and phosphate remover will have more impact. My bristlenose cleans the glass constantly (I don't have to touch it) but does nothing for algae on plants like hair algae....