A dogs poop that has mucus in it is usually pretty normal due to the way the dogs digestive system works.
The mucus that is usually present in the dogs poop is there to help the intestines and bowels stay lubricated to help prevent the dog from developing constipation.
The mucus in the dogs stool is made by the dogs intestines to keep the lining of the dogs colon lubricated and moist and is perfectly healthy for the dog.
Sometimes it may also be caused by something that has gone wrong with the dogs digestive tract as well if the mucus is more than just a little bit.
The mucus in the dogs stool can also be caused by something that the dog ate but a little bit of mucus in the dogs stool is perfectly normal.