You can brush of mold ... I've done it on several occasions, and smoked the cigar ... and it was still OK if it wasn't in the advanced stages
Mold is hairy looking, and will be slightly greenish/blue in color. Bloom/Plume is whitish dusty crystals that form when the oils are coming out of an aging cigar.
In other words:
Good:
"Bloom" or "Plume" (an extruded oil substance on a cigar) is the obvious result of a chemical reaction going on in a dark, moist atmosphere such as inside your humidor, causing the oils in the cigar leaf to come to the surface.
This fine whitish dust does no harm to the cigars. On the contrary it indicates that the cigar is ageing nicely and may improve in taste.
There is no need for alarm - it is both tasteless and odourless.
For the sake of appearance the bloom can easily be removed with a soft cloth, tissue or baby brush.
Bad:
"Mould" or "Mold" depending on whether you use the English or American spelling, is a signal for action.
It is evidence that the cigars are being stored in conditions that are too damp and/or too hot. Mold is distinguishable from Bloom by its bluish tint, musty aroma, and (if you smoke them) the mouldy taste. You'll recognise the "damp cellar" smell. If you try to brush it off, it leaves marks on the wrapper.
The best action is to separate and remove all the cigars from the humidor. This will allow the humidor to aerate and dry out. You can even wipe the inside with a dry cloth. Once it is clean and dry (check the smell) you may replace the unaffected cigars.
You may be able to rehabilitate and save the mouldy cigars but if not, discard them.
Do not put them in with the clean cigars. They can have a seriously detrimental affect on them, ruining your smoking enjoyment.
I use to know where there was some really excellent pictures someone posted of both examples somewhere on the internet ... but I can't seem to find them now. If I do ... I'll post the link.