Tidus did this too in his 'teenage' years. It looks very disgusting, and with Tidus he actually dripped it out on to the table, right at the vet's office. I freaked, and the vet told me it was completely normal for growing male pups to do this and seemed unconcerned. So I left it at that. And he was right, it never turned into anything. Hopefully that is the case with Guinness too!