Weird gray patches on dog's elbows?

My boyfriend, his friend, and I went on a nature walk at a local conservation area last night and we could hear a beagle after rabbits. He sounded really close, but suddenly my bf's friend says, "That's not a beagle." Stalking up the path, with the same intent on Sheldon that a cheetah has hunting antelope, is a HUGE black Great Dane. My boyfriend tried to be all macho and goes, "Everybody walk slowly and stand next to me." I, being a very doggy person, crouched down, clapped my hands together, and said, "Tum here boy!" and the dog's ears went back and he came right to me with his tail wagging. He and his beagle buddy accompanied us for about half of our walk before heading off for home (his tag indicated that he lived nearby) and I studied him. He had these weird gray patches on his elbows. No fur, and they were soft and squishy. I couldn't tell if it was skin and his owner had been rubbing lotion or something on them, or if it was some kind of like...mold growing on his skin...?