Moon Halos are pretty rare as I've only seen around 4 moon halos in my lifetime as of yet.
Moon Halos are caused by moonlight that passes through thin clouds of ice crystals that are high in Earth's atmosphere.
Then as the moonlight passes through the ice crystals, it is bent in a way similar to light passing through a lens.
That is then what causes the halo to be around the moon and it's pretty rare to see a moon halo so if you get the chance to see one have your camera at the ready if possible.
It's a pretty sight to see a moon halo and I wish I could've snapped pictures of them when I saw them.