Might be described as 'radiant'!
Moses is an Old Testament prophet who has not officially been canonized as a saint, but is definitely enjoying time with God, as he was seen in a glorified state alongside Jesus and Elijah at the Transfiguration. Therefore, instead of the traditional circular halo, he has little bits of light shooting out of his head. Why is it this way? Well, that's just the way things were in medieval times!
"Beatified figures, not yet canonised as saints, are sometimes shown in medieval Italian art with linear rays radiating out from the head, but no circular edge of the nimbus defined; later this became a less obtrusive form of halo that could be used for all figures."