If the moon was full, and now it’s getting slimmer each night, it’s a waning moon: it’s getting gradually smaller.
Waning can be used for anything that diminishes, like the brightness of the sunlight in the late afternoon, your interest in cartoons, or your influence over your teenage daughter’s fashion choices. It comes from the Old English verb wǫnian, which means “to lessen.”
a gradual decrease in magnitude or extent
(of the Moon) pertaining to the period during which the visible surface of the moon decreases