One of my cats got a dry spot on the ear once and it just cleared up by itself - not sure where it came from but the skin just flaked off. I figured it was probably just from a small scratch I did not notice at first.
Does your cat go outside at all? It could be ringworm which is NOT fun. Without seeing I don't know, but do a search for ringworm and if it sounds/looks like what your cat has, get to a vet ASAP.