[souffle]

It looks like a scab? Is there a scab on the area at all or is it pigment? It looks like when the fur gets stuck round the wheel spindle and ripped out or is rubbing on the wheel and it gets sore. Check there is no fur in the wheel spindle and that his back is not touching as he runs. Long hairs can get static and the hair stands up and catches sometimes.
If it is pigment and seems raised I would have it checked by a vet.