Mold vs. mildew — does the difference matter?
Clearing up a common point of confusion.
The short version
Mildew is really a surface mold — the flat, powdery growth you see on tile and shower grout. Other molds grow into and through material, which is why they are harder to remove and more likely to signal a moisture problem behind the surface.
If wiping it away keeps it away, it was likely surface mildew. If it keeps returning, something is feeding it.