Plant problems

I just started a tank about 4 weeks ago and put some live plants in. Now I am getting a dark brown or black coating on the leaves and rocks and an diver I have in it. I was wondering if plants are causeing the coating or is it something else in the tank? If you rub on the leaves it kinda comes off but I am lost on what it could be. I took the plants out in fear of it wrecking the rest of my setup or the fish.