Mate, that black stuff is a real pain. There's only a few types of fish that will eat it.not sure what types, but I know there hard to get. Pleco's I think.
If you don't want to use chemicals, you have to clean everything with the algae on it and change your tank water a lot,as in 20% a couple a times a week until it's gone. I actually got rid of all of my plants and replaced them with artificials and reduced the light time as the algae was taking over the tank. Big Time. I have a troop of eight Bristlenoses that hardly even touch the algae, they seem to be happy eating the big bit of avacado wood sitting in the tank.
You can clean your plants and rocks with very very diluted bleach.
Good luck and happy cleaning