I have seen something similar a couple of times in mice, but it doesn't match the ringtail link supplied and has never been too serious. All I did to clear it up was apply Johnson's tea tree skin cream for small animals every other day. This seemed to clear it up within a couple of weeks with no lasting effects and no loss of tail etc. Both times it happened in my mice it was to adult does, though this could easily be coincidence as this is obviously what I have most of, being a breeder.