The glyph we use for the Moon is the alchemy symbol for silver. It is also a drawing of a crescent moon (☽). The crescent element of an astronomical symbol represents the human mind or spirit. Partly in the light and mostly in the dark, it shows us how there are things in ourselves that are partly visible and known to the world, and parts of us which are not. Think of how there are great swaths of impulses, emotions, memories, and knowledge lying below the depths which our thoughts are unaware of, and cannot penetrate. That is what the Moon is, and what the symbol seeks to communicate.
Silver is a real metal and is connected to the Moon. It is one of the most versatile metals, one of the 3 base metals in Alchemy. It is associated with philosophical traits of intuition, self-reflection, and inner wisdom. It is a feminine metal, a symbol of purity and is connected to the goddesses and spirits. Its energies include divination, healing, protection, emotion, love, wisdom, dreams, luck and wealth. It is symbolic of attributes such as vision, clarity, awareness, focus, persistence and subtle strength.
The Moon is a card of illusion and deception, and therefore often suggests a time when something is not as it appears to be.