My daughter has one on her chest. She has had in since birth. It got bigger for a while and even had a pretty noticable knot in it for about a year. Her doc kept an eye on it, but once she realized it was not going to interfere with her breast tissue (it has moved higher as she grows and her skin grows and stretches). I think the doc said something like 90% go away by age 2, of those 50% by age 4, of those 25% by age 6. She is 6 1/2, and it is much lighter now, but may never go completely away.
As long as your pediatrician is keeping an eye on it, I wouldn't worry. It may not be pretty, but it is very likely it will go completely away. My daughter's has faded and stretched to where it resembles (to us) a crown, so we call it her princess crown and tell her God gave our little princess her own special crown, and she is proud of it, and not at all self-concious about it.
K.