What causes the smell of something being "musty"? You know the smell that arises when you leave a wet sponge out for too long? Does anybody know the chemical structure of the molecule that causes that smell? Similarly, what chemical process might be underpinnig that smell from occuring in the first place?
The "musty smell" is from mold, sometimes bacteria. Different compounds are responsible depending on the species. Some examples: trichloroanisole, tribromoanisole, dimethylmethoxypyrazine. Search "mold smell molecules" for many links.

Anisole is also called methoxybenzene, structure here: https://en.wikipedia.org/wiki/Anisole