If you see fur in the snakes stool, it's completely normal. However if the majority of the mouse isn't digested, then something may be wrong. Your bp might be dehydrated if he didn't fully digest the mouse. This may have been a one time thing, so pay close attention to your snake's next stool. If you're really concerned, get some gloves and see if there is blood or bones in the next stool. Best of luck to you, I hope I was a little helpful