The name "Phlegm" is not associated with Fleur in any way. It's possible you're thinking of a fan nickname or a joke that exists outside of the official canon.
However, it is true that some fans have jokingly referred to Fleur as "Phlegm" in online discussions due to a perceived similarity in the sound of the two names. However, this is purely a humorous, non-canon reference and not an official term.