Black mold is the type that will usually form on cement statues due to moisture. This type of mold as with any mold is dangerous if inhaled. The mold forms on the cement, then slowly grows, covering the statute completely if not removed. It will break down the composition of the cement, which damages the statue.
It grows on the surface or any other aperture of the statue and grows by living off the trapped moisture. As the mold proliferates it traps ever increasing amounts of water which, in conjunction with the excreted acids caused by metabolism, makes the cement brittle. If left untreated the statue will eventually desintegrate.
The only thing I've ever notice is discoloration of the statues. Once the statues have been cleaned
using elbow grease, a scrub brush and a little bleach, they will be as good as new. I have never
seen any damage as a result of the growth of mold on the variety of statues I've owned.