There is a fascinating post on the Kitchn today about vinegar mother:
Do you see it? That slimy red substance at the top of this ceramic jar is called a mother of vinegar, or vinegar mother. It's basically a combination of cellulose and acetic acid bacteria that forms on fermenting alcoholic liquids. As the name implies, a mother of vinegar makes vinegar!
When added to wine, cider, or other alcoholic liquids, a vinegar mother creates vinegar with a little help from oxygen. Mother of vinegar sometimes forms in store-bought vinegar bottles when some sugar or alcohol somehow gets into the vinegar when it was bottled. It shows up as a thin layer of slime floating on top of the vinegar. It's harmless and can be filtered out using cheesecloth or a coffee filter. Bragg's Apple Cider Vinegar comes with the mother.
Read the whole article at: http://www.thekitchn.com/thekitchn/food-science/how-to-make-your-own-vinegar-096977
You can even learn how to make your own vinegar!