The way I get rid of Black Beard Algae is to take out all the carbon from the filter / s
Double dose the tank as per instructions on the bottle of Florish excel and don't do a water change for at least a week.
I notice that all the black beard turns white then disappears.
Note: double dosing will kill your twisted val and riccia. but has no effects on discus, clown loaches, bristlenoses (common and golden) and guppies as well as kribensis.
Happy dosing.