My cat has runny poop with mucus after new food. Should I worry?

If you did a quick switch from the old to the new food this could definitely cause diarrhea, especially if no other symptoms are present. You can try going back to the old food until the stools form again and doing a slow transition to the new diet over a week. If the diarrhea occurs again this is likely not a good food for Olaf. Some animals with diet related diarrhea may benefit from a probiotic. A few commonly used in pets are fortiflora and proviable. Certainly if the symptoms do not get better, get worse, or are accompanied by blood in the stool and/or a decreased appetite/vomiting/lethargy a visit to your veterinarian is recommended. I hope this helps and thank you for using Petco Pet Education Center, formerly Petcoach!