Black Beard algea?
You have plants that are not very nitrogen/nutrient dependent, resulting in a very nutrient rich water column, I would do more water changes. The rocks could also be leaching minerals depending on pH.
Consider adding riparium plants, reducing photoperiod, raising lights higher above plants.
I had quite the BBA problem until I got the Siamese Algae eaters in the tank. They did a very good job of reducing that to nothing. I'm not sure what kind this is. It is still black but I haven't really tried to clean it off. Any recommendations on plants to add that will reduce nitrates?