Perusing Instagram or any cigar blog you'll eventually run across a plume vs mold argument. Its usually posted by someone scared of what they just saw in the humidor. They wonder is this cigar safe? I must go to the elders of the village and find out the nature of this evil taking over my cigar. Will it spread to the other cigars in my humidor? If I smoke it will it kill me?
Fortunately there are only two possible explanations for that strange stuff you your cigar, its either plume or mold, but how can you tell? Firstly, lets explain what they are. Plume comes from aging the cigar to the point that the oils start to pierce through all layers (wrapper, binder, filler) of the cigar and leave dusty looking deposits. Many would-be aficionados will claim that this is the result of a humidor that is too humid and too warm but I disagree. For me its a sign that the cigar is ready to smoke with the right amount of maturity. On the other hand, mold is a fungus that grows in spores and can be very dangerous. Its usually yellow, green, or blue-ish and grows in clusters. Unlike plume molds does not spread uniformly; instead, it grows in clusters.
The best way to determine the difference between mold and plume is to first try and blow it off. If it blows off it is definitely plume. The second test is to try and rub it off. If it comes off easy its plume but if it sticks it is almost always mold. Lastly, plume will usually take place along the entire body of the cigar and not in one area.