Black algae, or commonly called either black beard algae of black brush algae, is a type of algae that will appear to grow in a bush like fashion inside of the water. It will have a black appearance if the algae is healthy, and a red or orange appearance when the algae is starting to die off. For this reason, when it is dying off many may call it red algae, red hair algae, or red bush algae.
Black algae can easily be spotted as it tends to grow on objects within the water and not the tank or panels at all. Normally it will grow on things that do not frequently (areas with lack of full water circulation, slow growing plants, decorations that shield sections to produce dead spots, etc.) and will have the black color if the algae is heathy and growing up to it's speed. When it runs into a lack of nutrients it will turn red-ish orange while finally turning grey when it is fully dead and may start to peel away (giving the appearance of a bush or hair like).
Black algae is normally find in setups that will have live plants, since the added fertilizers required to keep many of the plants alive and healthy will benefit this algae greatly. This can be due to the fact that there are excess nutrients being added into the water column to feed the plants, the type of substrate that is being used (such as a clay based substrate, any treated, or naturally occurring substrate that releases nutrients into the water column) and in some cases if the setup has any soil either being capped by a layer of substrate or is not capped. Overdosing or having an unbalanced fertilization schedule can provide this algae with an environment in which is can thrive under (and is the common reason why many plants will die off since the black color of the algae fully prevents any light from reaching underneath it to the plant's leave(s)).
Another type of setup that is commonly prone to this type of algae will be one that has extremely low pH (it seems that due to the low pH it allows for this algae to grow and feed on whatever is being provided to the water). Due to the black color of this algae many do not consider it to be inside of the red algae family (until it starts to die off and shows its red-ish orange color before turning grey).
The prevention of this algae can range from first understanding what the root issue(s) are inside of your setup. In many cases making sure that you are not allowing for the algae is thrive inside of your setup is key and will slowly kill it off in time. To remove the algae you can simply remove it with your hands using a new toothbrush (since when it grows it will be extremely hard to remove unlike other types of algae). If it is on any parts of the tank or setup equipment you can try to use a magnetic scrapper or a sponge that has a rough surface on it (making sure it is a new sponge to avoid any possible chemicals being added into the water column).
In many cases the common way to remove this algae is to do what is known as a spot treatment, where one will pour hydrogen peroxide directly on the algae (using an eyedropper if you cannot remove the element that has it growing on it) while leaving all of your filters off for an hour or two, wait 5 minutes after pouring the hydrogen peroxide onto the algae (making sure to only use an extremely little amount (you want a maximum of 1ml per 8 gallons (30 liters) of water) if using 3% hydrogen peroxide, and then finally making sure to do a 70% water change if possible. After the water change make sure to provide an increased amount of aeration to the water as hydrogen peroxide will soak up some of the oxygen within the water which can make the fish starve from an oxygen depletion. Using this method rarely is a good idea since if you are doing this commonly it will have an extremely negative effect on all of the livestock within your setup to the point of being fatal, and can cause many plants to fully melt.