As stated, plume is the crystallization of sugars or oils from the leaf that have collected on the surface. Plume develops under certain conditions of temperature, humidity, and time that are favorable to cigars. Mold, as you know, is microbiological growth that also develops under certain conditions of temperature, humidity, and not always so much time, that is unfavorable for cigars.

It is debatable whether plume actually contributes to the enhancement of a cigar, or if it is merely an indicator of a well aged cigar stored under optimal conditions.