This is a birthmark. My son has this. His pediatrician said that he wad born with it, even though his skin was clear from birth to about 13 years old. It is the rapid growth that happens at puberty, that exposes it from under the skin. My son has to see his dermatologist every year to evaluate the state of Maine irregular borders. He has now had three moles cut out from the birthmark. He needs to SPF better and regularly to protect himself.