According to Marvin R. Shanken's Cigar Aficionado's Pocket Guide (1997 edition):
"BLOOM - A naturally occurring phenomenon in the cigar aging process, also called plume, caused by the oils which are exuded during later fermentations. It appears as a fine, white powder and can be brushed off. Not to be confused with cigar mold, which is bluish in color and stains the wrapper.
MOLD - A potentially damaging fungus that can form on cigars stored at too high a temperature.
OIL - Oil is the mark of a well-humidified cigar. Even well aged cigars secrete oil at 70-72% humidity, the level at which they should be stored.
[I can't help including this one as well: "PERIOD OF SICKNESS" - A time when cigars should not be smoked. Fresh cigars are fine, as are aged ones; but avoid cigars between three months and a year old. (No reputable store would sell you a cigar of this age.)".]
+ + + + + + +
Now we turn to Richard B. Perelman's Perelman's Pocket Cyclopedia of Havana Cigars (Second Edition, 1998):
"Aging of [cigars with strong flavors] will result in a mellowing in the strength of the cigars as the fermentation process continues. During this period, the internal oils of the leaves will migrate towards the exterior and will leave the wrappers oily and aromatic...Cigars which show white mold, also known as "plume" should simply be wiped clean with a barely-damp sponge or a soft cloth. If the mold has turned green, the cigars are ruined."
+ + + + + + +
Note how Shanken and Perelman are at odds: Marvin says plume is caused by migrating tobacco oils and is not mold. However, Rich states that plume is white mold.
By the way, has it been established scientifically that tobacco oils move themselves to the surface of the cigar, or is this one of those urban myths associated with cigars? Do cigars actually "ferment" as they age? I could pose some more questions, and make a few comments, on the above extracts, but will leave it at that.