This could be either a worm infestation, such as roundworm, tapeworm or hookworm, or it could be parvo or corona (a vaccinated dog can pick these up, but to a lesser degree) or a case of colitis (irritable bowel syndrome).
Keep your dog away from food for the next 24 hours. Make sure it stays hydrated, and if it won't drink water, give it a few tablespoons every hour. After this time, give it a small meal of boiled chicken and rice, which is easy to digest. If there is something in the dog's stomach that is not a worm or virus, it will get wiped out as the stomach empties, and when it eats the chicken and rice meal, which is easy to digest, it will stay in its stomach.
If this does not resolve the problem, you need to take the dog to the local vet for a test. Make sure you take a stool sample with the mucus with you for testing. They will either give you a de-wormer, or treat the dog for another illness that you cannot treat at home.
Keep your dog away from food for the next 24 hours. Make sure it stays hydrated, and if it won't drink water, give it a few tablespoons every hour. After this time, give it a small meal of boiled chicken and rice, which is easy to digest. If there is something in the dog's stomach that is not a worm or virus, it will get wiped out as the stomach empties, and when it eats the chicken and rice meal, which is easy to digest, it will stay in its stomach.
If this does not resolve the problem, you need to take the dog to the local vet for a test. Make sure you take a stool sample with the mucus with you for testing. They will either give you a de-wormer, or treat the dog for another illness that you cannot treat at home.