I just talked to the surgeon, and she said that Selena appears to have a venous malformation, which is an abnormal collection of veins. She was born with it, and will always have it. It doesn't appear to be compressing any nerves or interfering with anything else, so we will just watch it and make sure it doesn't become symptomatic.
The only downside to it not being a hemangioma is that it will not disappear. She will always have a slight asymmetry to her shoulders. But we are so thankful that she won't need surgery!!
I would run and give her a big hug, but she is napping :-)