Is mold natural?
Mold or mould, however you prefer to write it, is part of the Fungi kingdom. It represents roughly a quarter of the Earth's biomass material. So it qualifies as being in it's natural environment. Cohabitation with other living organisms is either beneficial or detrimental, and some times it can be symbiotic (serving both parties).
Mold plays a vital role in nature with bacteria in decomposing organic mater, recycling them in a sens, creating in this process earth material suitable to harvest new life and environments for other living organisms.
So in a way mold, fungi and yeast are essential parts of Nature.