This aquarium algae belongs to the red algae family and it usually grows on the edges of plant leaves and aquarium wood and rocks, forming very dense patches which look like a beard, hence the name BBA (Black Beard Algae).
Its color varies from bright green to blue-green, all the way to a more darker green.
Among the aquarium inhabitants, the Siamese Algae Eater (Crossocheilus oblongus) enjoys nipping at it.
Generally requiring a weak bleach solution (!) to kill it, this is one of the most difficult aquarium algae to remove. It is usually first noticed on plants but it can extend on hardscape materials like rocks and wood.
Causes
Unstable or low levels of CO2. Contaminated plants that are introduced in the aquarium.
Prevention
Soak new plants into a solution with 10% bleach to kill off any trace of BBA, before placing them in the aquarium.
How to remove
Gently brush off the algae from plants and aquarium surfaces like rocks and wood. Do more frequent water changes. Introduce some Siamese Algae Eater fish in the aquarium.