The meaning of this word seems to have something to do with how scary they may look in dimmer lighting, as you can only see their pearly white's in the dark.
A spook is a type of person that rids a place from monsters like boggarts, ghosts, witches, and more. This paranormal person keeps people safe and help the dead pass to the other side. This profession is from my favorite series: The Last Apprentice.
The spook carried a staff and dressed in a tall black cloak that masked the wearer’s face.