There are wet towels the shower of my bathroom. They have been there for the past three days, and yesterday it started to smell stale-like. I don’t know if this is mold, mildew, or anything else harmful. I can’t see any mold stains. Can somebody please help my answering the following questions:

Is what i described Mold, Mildew, or anything else?

If I wash these towels with the rest of my laundry like normally, will it spread to the other clothes?