It is really hard to tell from the photo. Is it hard and appears to be stuck on? If so then it is likely to be "boar glue" (i.e. semen), which in piggies sets rock hard. This is a common place to find it (as well as the nose) as they clean themselves. You can try soaking it off, or it will drop off by itself. If it isn't boar glue and you are worried, take him to the vet to put your mind at rest.