Anne McCaffrey's the Unicorn Girl

She was just a little girl, with a tiny horn in the center of her forehead, funny-looking feet, beautiful silver hair, and several curious powers: the ability to purify air and water, make plants grow, and heal scars and broken bones. A trio of grizzled prospectors found her drifting in an escape pod amid the asteroids, adopted her, and took her to the bandit planet Kezdet, a place where no questions are asked and the girl might grow up free.

But Kezdet has its own dark secret. The prosperity of the planet is based on a hideous trade in child slave labor, administered by "The Piper" -- a mystery man with special plans for Acorna and her powers. But free little girls have a way of growing into freedom-loving young women, and Acorna has special plans all her own.