Two different white substances can appear on cigars, and they should not be confused. Plume (also called bloom) is a dry, powdery film, said to be crystallized oils, that wipes off and does not smell musty. Mold is a living growth: fuzzy, sometimes blue-green or gray, found on the foot or the wrapper. A moldy cigar should be thrown away.
People in the trade disagree about how often true plume occurs, and this page does not try to settle the question. The practical test is texture and smell. A powder that brushes off a well-kept cigar is one thing; spots with thread-like filaments are mold.
Mold is caused by a humidor problem: too much moisture, no air movement, or an overwet humidifier sponge. Keep the relative humidity within the commonly recommended range and open the humidor now and then to let air in. See humidors.