Can mold grow in vinegar?
Normal vinegar concentration (usually around 5% acetic acid) is too acidic to grow mold in the vinegar itself. Mold can sometimes grow on the bottle or on the surface of the vinegar. It isn't dangerous and can be wiped/skimmed off.

What you are seeing is called the mother.



Vinegar is produce when acetobacter bacteria consume alcohol and produce acetic acid. The bacteria form a culture as they work. In the case of filtered vinegar, this culture is removed before bottling but letting a bottle of unpasteurized vinegar sit for a while will allow the culture to reform. Unfiltered vinegar often already has some of the mother present.

It is harmless and can be used to start your own vinegar if you have stale wine sitting around.