My son was born with a patch of white/gray hair in the back of his head. We were told it was a birth mark as well. It's totally cool.. he prefers to wear his hair longer (he's 5) and has this white strip of hair in the back that shows when he's running around. He's aware of it and thinks it's cool.
I think that if your dear daughter gets the message from you that it's okay and unique and cool, she'll feel the same way. Kids are always going to be cruel and if it isn't about her hair, it might be about something else. Our job as parents is to help them feel confident no matter what anyone says to them and be compassionate when they're feelings are hurt.