Looks like Black Beard Algae. How long do you run your lights daily? What kind of lighting and bulbs are you using? Do you use CO2 or liquid carbon? What ferts do you use?
For starters only run lighting 6 hours until algae is gone. You can also spot treat the BBA with hydrogen peroxide 3%. Turn off your filters, pull up 1-3ml of peroxide for every 1 gallon of tank water, hold syringe as close to the algae as possible and slowly squirt with the peroxide. Leave filters off 20 minutes. If you have a lot of BBA you'll have to do an area a day. Also usually within 24 hours you'll see the BBA turn white, pink, or red which means it's dying.
Just remember you need to fix the problem causing the BBA or it will keep returning no matter how much you treat it.