If they are kept in the freezer and the spots are there when you take them out then it's freezer burn. The longer they are in the freezer the more freezer burn they get. I have had mice and rat pinkies in the freezer since this time last year and I am throwing a lot away due to freezer burn.
It probably doesn't hurt the snakes but my personal habit is that if I wouldn't eat it then I won't feed it to my animals. And, yes, if mice and rats were all that was left in the world to eat I would eat them.