What troubles Pearl as she sees her reflection in the pool?

Pearl felt her feflection represented shades of someone else: a different girl. She thought it to have strange qualities.

Just where she had paused the brook chanced to form a pool, so smooth and quiet that it reflected a perfect image of her little figure, with all the brilliant picturesqueness of her beauty, in its adornment of flowers and wreathed foliage, but more refined and spiritualized than the reality. This image, so nearly identical with the living Pearl, seemed to communicate somewhat of its own shadowy and intangible quality to the child herself. It was strange, the way in which Pearl stood, looking so steadfastly at them through the dim medium of the forest-gloom; herself, meanwhile, all glorified with a ray of sunshine, that was attracted thitherward as by a certain sympathy.