I notice mold likes to grow on silicone caulk. I find it odd since silicone is mostly inorganic and doesn't contain any nutrients for mold to grow on unlike soil and clothing. Can mold metabolize silicon?
This product brochure seems to give a reasonable explanation:
The following conditions help to form mould on sealants:
High air humidity with little movement of air, eg: in bathrooms, showers, kitchens etc.
Nutrients in the form of organic deposits. e.g. residues from body care materials such as soap, shower gel etc.
Since elastic silicone joints have low thermal conductivity they are the warmest part of a tiled surface and where mould thrives best, in combination with organic residues and moisture.
The EPA has outlawed certain fungicides from time to time ; it takes time for manufacturers to develop new ones. I have caulk around some windows that has turned black with mold but a nearby window with different caulk is still white . The windows are about 10 years old . 25 year old original caulk is still white . (These are all exterior exposures). It depends on the mold inhibitors available at the time.