Black beard algae (also called BBA and brush algae) is botanically a red algae. If you put red algae in alcohol, they will turn reddish. This is an easy proof that the algae you’re dealing with are actually red algae.
Black beard algae grows on the edges of plant leaves as well as on the edges of almost any hard surface. Composed of very fine strands or tufts, it grows in dense patches resembling a dirty dark almost black green beard, thus earning its name of black beard. It can also be bright green, blue-green to blackish green or even purple. It is soft, slippery, and grows rapidly, yet clings tenaciously to plants and cannot be easily removed by hand.
It generally comes with high nitrates, high phosphates or high iron. So heavy duty water changes can often control it.
Here is a link to an article on controlling algae:
16.2. Controlling Algae
Startpage Aquariumscience
Source: Aquariumscience.org – David Bogert
