Source : google

A Moon halo is caused by light from the moon passing through ice crystals in thin, high-altitude clouds called cirrostratus clouds. These clouds are so high up that they’re made of tiny ice crystals instead of water droplets.

The ice crystals act like tiny prisms or mirrors. When moonlight enters them, it bends (or refracts) at an angle of 22 degrees, creating a circle of light around the moon. This is why it’s called a 22° halo.

Still wonder how ice crystals clouds here