Sounds like intestinal mucus. Mucus is found in all stool but usually we cannot see it alone since it "coats" stool so it can leave the body more smoothly. When mucus is present alone it usually points to inflammation in the intestines. This can mean eating something you have a sensitivity to, or something as common as constipation.
It has a fishy smell by itself, but if it touched stool then it can be responsible for that fishy sewage odor.
Your bowels are probably backed up. Treat the constipation, make sure you drink enough water and get enough soluble fiber everyday, and the smell and leakage should go away.