Just a thought, but is the greasy stuff being thrown out from over greased wheel bearings or a failed seal?
Are the discs discoloured? If so the brakes may have been binding.
Use brake cleaner for the discs, it's designed for the job and won't leave any kind of residue. Believe it or not, washing up liquid and very hot water is pretty good too, just rinse the parts of thoroughly afterwards.
Hope this helps
Rich
