Black beard algae.
Its not harmful to fish, but it does tend to take over a tank and if it covers over plants the plant will die.
Most people find it unsightly and dont want it in their aquariums, but some people like the look of it.
Usual cause is too much light. Either too intense, too long, or both. Cutting back light will control the BBA over a long time. There are chemical treatments, like an overdose of a CO2
booster product (Seachem Excel) which is actually a mild algaecide called gluteraldhyde. Turn off your filtration, get a syringe, measure out a dosage for your aquarium, spot apply the excel to some of the BBA, leave it 5 minutes for the excel to work then turn on the filtration again to dissipate the excel about the aquarium. It should kill off a little of the BBA in the area you applied. 24/ 48 hours later repeat the process on another patch of BBA. If a normal dose isnt having the desired effect then double dose from the syringe.
Some people double dose the whole tank, daily, over several days rather than spot applying locally. But, see the disclaimer below.
. Gluteraldhydes normal use is to sterilse medical equipment. Its harmful to fish. You have to be careful with dosage and its a fine line between no harm and kill everything. Killing BBA isnt the purpose of the product, using it in such a fashion isnt recommended by the manufacturer and increasing the dosage isnt in line with the manufactuters instructions. Even with a normal dosage spot applied locally, if a fish swims through the treated area before the filtration is turned back on to dissipate the product, it can harm/ kill the fish.