Answer: Personification.
Explanation: In this stance we instantly see she is using personification to describe the moons actions. “Slowly”, “Silently” as the words she chooses to describe her movement through the sky. It’s also important to note that Walter De La Mare refers to the moon as a “she” implying that she has her own thoughts, her own feelings and at this moment her slow progression through the sky was her owning choosing.
This is a classic example of personification.