Guinea pigs can get lice or mites. There are so-called "static lice" that look like tiny black dots on the shaft of the hair, but these usually appear around the rump and back legs. From a distance on a white guinea they make them look dirty, but when you look closely you can see the sparate dots and they can't be brush off easily. If this is on the face, it's unlikely to be static lice. It could be dirt, in which case should be easy to remove with water, or it could be "freckles" or similar. If you are worried, get a guinea pig vet to have a look