Why there is a rainbow ring some time appeared around the moon ?

Refraction of light through ice crystals.  https://en.wikipedia.org/wiki/Halo_(optical_phenom...

A 22° halo is an optical phenomenon that belongs to the family of ice-crystal halos. Its form is a ring with an apparent radius of approximately 22° around the Sun or Moon. 

Because of ice crystals in the atmosphere acting as prisms.

Atmospheric refraction, usually from ice particles in the upper atmosphere.  Refraction is when light bends (changes direction of movement when passing through an interface or interfaces between substances), like how a prism works.  Volors separate because the amount of bending at the interface depends a bit on wavelength, and colors exist because they have different wavelengths.

In effect, lots of ice crystals act like tiny prisms. If you have enough of them, we can see that effect down here.  

Need to teach you some basic optics (wave behavior) to say more.

The ring, or halo effect, is caused by the refraction of sunlight through ice crystals in the upper atmosphere, around 3-6 miles above the Earth. Usually the ring is seen along with cirrus clouds, the thin, wispy clouds seen at high altitude.

When a leprechaun hides their pot of gold on the moon, it causes a rainbow ring around the moon. 

Gay aliens on the dark side of the Moon. 

Its called a halo and is the result of moonlight refracted by ice crystals.

https://en.wikipedia.org/wiki/22%C2%B0_halo

Humidity in the air like fine water drops. That refracts the light like a prism or true rainbow.

Ice crystals in Earth's atmosphere.

http://www.snowcrystals.com/halos/halos.html