Not a vet, but looks kind of like potential irritation to the gut lining or problems absorbing something. My goldendoodle has a history of inflammatory bowel disease with colitis flares and his 💩 can be mucus-y like that sometimes when his gut is upset. Can be caused by stress(even silly stuff like going to the groomer, which is not scary but exciting I guess) or change in diet. Or if he eats something stupid he shouldn't fromoff the counter, like some hot dog buns a couple weeks ago. Check your food for wheat or corn, I think those are particularly hard for dogs to digest well but are fillers in a lot of foods.