In scientific terms, mildew is a specific kind of mold, usually white, that occurs on plants. However, most people think of mildew as the mold that grows between tiles in the shower—obviously, the bathroom is a high-moisture area and you’ll likely see some mold growth on tile or grout from time to time. This is generally not cause for concern unless you don’t clean it up or the room has inadequate ventilation and mold growth spreads to drywall or trim.