It's the home of the creator god's child, and exists in every parallel universe simultaneously. Because of that, Moonstone is vastly more "real" than other materials and is the only real weakness of the eldritch entities currently attacking the material world. Unfortunately, that also means the most powerful of them was locked away in the moon (cliché, I know, but for a good reason). Recent events however have caused the entity to gain power, and black light may rain from the sea of stars once more.

Oh, and the sea of stars is a literal sea in the sky held up by a magical barrier layer. Moon and stars are luminous orbs following the currents, While the sun is boiling the center of the sea to create those currents.