The black gunk in the moss? I get that and hair algae all over if I neglect regular maintenance.. it likes moss swords and crupts, healthy stems seem to shake it pretty easily.
Maybe remove the moss and treat it with peroxide or glut, or salvage some of the moss and grow it DSM style in a tupperware or gallon baggy. Replace the moss later.
Debris in the water really kicks mosses in the butt, making them more vulnerable to algae infestation.
In my tanks, physical removal is about the only thing that works.