The words get used interchangeably. They shouldn't be.
Mildew
Mildew is surface-level fungal growth. It's the powdery white-to-gray film on grout, shower caulk, leather, and the underside of a wet leaf. Botanically, "mildew" is a specific kind of fungus. In housekeeping, it gets applied to any flat, surface-living mold on a hard surface in a damp area.
The important property of mildew, as people use the word at home, is that it lives on the surface. It hasn't penetrated the material. A bleach-and-water spray with a brush usually finishes it.
Mold
"Mold" covers thousands of fungal species. Many of them colonize inside porous materials - drywall, wood, carpet pad, insulation, ceiling tile, books. Once mold is inside a porous material, you can't surface-clean it back to safety. The visible part is the fruiting body. The body of the colony (the hyphae) is woven through the material.
How to tell which you have
| Signal | Likely mildew | Likely mold |
|---|---|---|
| Surface | Hard, non-porous (tile, grout, caulk) | Porous (drywall, wood, insulation, carpet) |
| Color | White, gray, light tan | Black, dark green, brown, with depth |
| Texture | Flat, powdery | Fuzzy, slimy, or with visible texture |
| What's underneath | Clean | Stained, soft, or compromised |
| Smell | Mild | Strong musty/earthy |
| Wipe test | Comes off cleanly | Leaves a stain or pulls material with it |
The cleaning rule
If you can wipe it off a hard surface with a household cleaner and it doesn't come back within a week, it was mildew or surface dust. If it comes back, or if it's growing on or through drywall, wood, or insulation, surface cleaning is not the answer. The colony is inside the material. The only fix is to remove the material.
That's the line where you stop reaching for a spray bottle and book a real inspection.