Plume appears as fine, even, crystalline dust — it brushes off cleanly and leaves no mark. Mould is fuzzy, often bluish or green, grows in raised spots, and leaves a stain when removed.
The honest position
A substantial number of photographs posted as plume are mould. The safest test is texture and colour, not hope: crystalline and white is plausible, fuzzy or coloured is not.
What to do
A mouldy cigar should be isolated immediately and the humidor inspected, since spores spread. Humidity above roughly 72% is the usual cause.