The wood in humidors, as well as cigars absorb moisture so there was water in the humidor.

Investigate the cigars when you get the humi. Luckily I've never had mold on one of my cigars. Nor plume(bloom)...one day... Anyway, in my research I've found that plume will be a light uniform dusting on the wrapper of the cigar(s). Mold will grow in patches and you might be able to see "stalks" of the mold if you look closely. Read me.

It sounds like it is mold to me, but you will have to see for yourself. If there is no mold near the foot(the open lighting end), trash em. You can assume that mold got inside. If it's not by the foot, you might be able to salvage them.

From cigarhumidors.vigilantinc.com:
Gently wipe the mold off of your cigars and leave them at room humidity for 36 hours, then place them back into your humidor (obviously after addressing the moisture problem that caused the mold in the first place). Check these cigars every few days to ensure that mold growth has stopped. Some people recommend more drastic measures such as placing your cigars in the freezer to kill the mold. Try our method first before going to such extremes. If your problem continues, you may need a new humidor. (click for full Q&A)