Molds are naturally occurring biological organisms.
There are literally thousands of types of mold in the environment. The good news is that over the centuries, molds have helped produce some wonderful inventions such as penicillin, breads, cheese, and particular alcoholic beverages. The bad news is that molds reproduce by sending out spores. These spores activate and grow when they land on a moist spot. This mold then grows more spores and so on. The mold spreads and spreads.
Outside this is not necessarily a bad situation because mold is part of the natural process of decay. It is part of the reason that leaves that missed the fall raking will have deteriorated into soil by spring. However, when mold intrudes into a home due to an existing moisture problem, it can get ugly. Mold when left unremediated, can produce a strong musty smell, have a furry appearance, and be mushy in texture. Not an attractive sight.