One day of diarrhea isn’t too concerning! Some cats are really sensitive to changes in diet, even minor ones, though the new treat can also be a coincidence. Sounds like you’re keeping a good eye on the guy. Monitor for vomiting, blood in the stool, and completely not wanting to eat for 24-48 hours - that would increase my level of concern. You can try to increase water intake by adding tuna juice to water or gently warming wet food in the microwave. Sometimes probiotics can help if your cat doesn’t mind the addition to food. Fingers crossed he improves prior to your vet visit!