Shellies believe that a side effect of flying at the speed of light with their intergalactic suit, causes light to kinda... sticks to you? cuz now they seem to be glowing.
In order for something to glow, it needs to have a material called phosphor. When the atoms inside these materials get energy from visible light, they become “excited” and causes them to emit tiny packets of light called photons, which makes them glow. Shellies are also excited that they can go trick or treating without having to carry flashlights, wooooohoooo!!!

- scienceabc.com