Pearl, who is Hester and Dimmesdale's daughter, has not been acknowledged by her father neither emotionally, nor physically. For this reason, her role in the novel until the moment that she is recognized by Dimmesdale, is to basically wander through Hester's life as an "elf-child".
In the puritanical world, being an illegitimate child entails that you are half-human, for you cannot claim your full ancestry. Nathaniel Hawthorne cleverly uses this fact to bestow upon Pearl what seem like supernatural sensitivities that respond to the guilt of her parents. Up until the moment that she is publicly recognized, Pearl is a symbol of sin and of fate that serves the purpose of "reading" her parents' hearts.
The part in Chapter XVIII where Hester is in the forest looking to speak with Dimmesdale would be the first time that the "family" would be together. This would have been a shock to Pearl who lived her life beside her mother and was her sole companion. Like both, an ethereal and rudimentary "creature", Pearl detects the presence of Dimmesdale and feels a supernatural tendency of fear, dislike, and distrust. The expression of being "slowly" approaching is meant to resemble the reaction of an innocent animal in the forest which perceives the presence of something dangerous.
We know that Dimmesdale openly declares how children just do not connect with him, nor he with them. The implication of this is that Dimmesdale does not have the saintly innocence to which children would be attracted; he is anything but a saintly man. Pearl, in her supernaturally-sensitive heart, can feel this too and, until her "curse" ends, she treats Dimmesdale as her enemy.