" Look at meaning #1 for "slip" as a verb below. com/definition/english/slip
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Rover_KEThere was no reason for the author to capitalise 'phlegm'.I was thinking about that. I haven't read (and never will read) Harry Potter. I assumed it was being used as a proper noun for some reason.
contrapositionFleur Delacour is called Phlegm in this passage and it seems that a pun is intended.Aha!