It's probably the poop from your fish that you just bough at Petsmart last week. Unfornutaley, you forgot that fish poop and that they can be very difficult to clean
If it is within the rocks or floor of the tank it is most likely fish excrement. Check to make sure that you are not over crowding your tank. An over crowded tank can create stress on the fish. Also, if you are over feeding your fish, this will cause the fish to increase waste production.
What you have sounds like an algae problem. You can control the growth of algae by limiting the lighting your tank gets, as well as doing partial water changes.
I've frequently had a problem with black "slime" in my aquariums over the years. It's a form of algae, and it seems to spread quickly once it appears. If you are looking to get rid of it, you will need to clean your tank and balance the PH.
The black stuff is Beard Algae and is very hard to get rid of. You need to clean your take completely and soak all plants and any gravel in a beach solution then scrape the algae off of them. For the next month you need to do at least a 25% water change, and limit light.