The Moorchild

From Amazon:
Half moorfolk and half human, and unable to shape-shift or disappear at will, Moql threatens the safety of the Band. So the Folk banish her and send her to live among humans as a changeling. Named Saaski by the couple for whose real baby she was swapped, she grows up taunted and feared by the villagers for being different, and is comfortable only on the moor, playing strange music on her bagpipes.

As Saaski grows up, memories from her forgotten past with the Folks slowly emerge...