That is Black Beard Algae. How long are you running your lights daily? What lights are you using and bulbs? What is your nitrate level? Also are you dosing any ferts?
Often BBA is caused by running lighting too long. But that is not the sole cause. Try running your lights only 6 hours daily until algae is gone for starters. You can also spot treat the algae with hydrogen peroxide 3%. Turn off your filter, pull up 1-3ml of peroxide to every 1 gallon of water in a syringe. Hold syringe right up to algae and slowly squirt. Leave filters off 20 minutes. If you have a lot to treat you'll have to do an area daily. Usually within 24 hours the BBA will begin turning white, pink, or red which means it's dying. You can also remove any rocks or other removable items and treat outside the tank.
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