Cause of puppy's diarrhea?

We just got our 9 week old Gordon Setter on Thursday morning, and he's been great. He's very energetic, alert, and a healthy eater. However, yesterday and today, he's been getting some intervals of diarrhea, and then normal bms. It's been very mucousy and kind of frequent. And earlier, I noticed a very tiny amount of blood, like less than 1 drop worth. We've been monitering him very closely, and he's still energetic, napping normally, and eating and drinking... He got into some dried celery and chex mix yesterday, I am wondering, what could it be? Should we take him into the vet asap?