What this video covers
Mold and mildew get used interchangeably, and the clip separates them.
Mildew stays on the surface. It is powdery, usually white or grey, and it wipes off with a household cleaner without leaving damage behind. Mold behaves differently, which is why the identification matters before you decide how to respond.
The takeaway
If it wipes away cleanly and the surface underneath is sound, you were probably dealing with mildew. If it returns or the material beneath is soft or stained through, treat it as mold.
Read the full written guide: How to Recognize a Mold Problem.