Is something wrong with my guinea pig?

As long as he's still acting normal and you've only seen this stuff a couple of times, I wouldn't be too worried. If it continues for more than a week, call your vet and ask if you can bring a sample, just to be sure. They will either ask you to come get a sterile container or you can use a plastic bag or container. Better to be safe than sorry with this kind of thing, and they could probably just run a check for parasites to be sure that he's alright.