Greasy black residue from back rear tire inside of rim.

The vehicle drives but brake pedals almost hit the floor and somewhat begin to break. And the back rear inner rim has black greasy residue, I do not think it has any odour or anything the rear back rotor is covered with that greasy residue, the driver side tho rotor is silver and hot to the touch and makes some sort of sound of like a grind. But on the passenger side rear that rotor is not even hot when I stop from driving a distance but is covered in the greasy black residue? Im not sure what it could be as I am not mechanically inclined I have been told by other it could be Calliper. Or break cylinder or wheel bearings, also my brake warning light stays on