Black Beard algae.
A number of things can help. Maybe a combination will get it beat down.
Too much light is possible.
Too little flow throughout the tank overall (though that said, I have also had a patch grow in front of my FX-5 out put which runs 300-400 gallons per hour through my 72G
issue being lower that it could be.
Treating with Excel in the whole tank regularly, providing it will not hurt the inhabitants, excepting the BBA
But just to get it out, you can trim off problem spots on the leaves, get all the pieces as it can spread. BUT you can also use CO2O household 3% hydrogen peroxide.
My recommendation would be to remove the DW and use something to scrape of the big areas like a gift card edge or corner or cut to the shape you want / or something else you feel comfortable using.
Take a cheap new toothbrush and bottle of Hydrogen Peroxide 3% and brush it all around, drip some on with a dropper, let sit an hour or 2.
Meanwhile, then get busy with the leaves. You can trim the sides and ends, by keeping it in the same leaf shape if you are trying to keep the leaf or pinch / trim the leaf off.
If you can remove the problem items to treat, it would be best inmho. There are plenty of Youtube videos of treating in the tank. This can get out of hand if you have lots of spots needing H2O2. It can cause death of inhabitants if there is too much H2O2.
For leaves use 50% water 50% H2O2. You can brush on, or try a magic eraser very gently, or a little piece of microfiber cloth or terry towel