Parasites such as the Hookworm are a common cause of a dog pooping out blood along with mucus.
The hookworm or some other worm parasite is inside your dog eating at it's intestines which is causing the dog to poop the blood and mucus.
You should take your dog to the vet to have a test done to see if the dog has Hookworms or other parasites inside it's intestines.
The worms if not treated in time could actually cause your dog to die so it's important to get it checked out because blood and mucus in your dogs poop is not normal.