A fusspot is a person who worries and complains about small, unimportant things: the cup not matching the saucer, the towel folded the ‘wrong’ way, a tiny mark nobody else can see. It is informal, a little teasing, and a little unkind if you say it to someone’s face — more often you say it about someone. It names the person, not one single act. A parent, a teacher, or a colleague can be a fusspot; the word is not as sharp as control freak, and not as medical as obsessive. British speakers use it freely; learners will also meet fussy person, which is the same idea in plainer clothes.