system
Given the following post, respond as if you're commenting on Reddit:
human
What's the white stuff that forms on batteries after a while?
gpt
It is mostly lead sulfate, caused by the reaction of the sulfates in the battery itself with the terminals.  It it forms, it means that the plastic sealing is not entirely perfect, although a small amount of it isn't cause for concern.  It is somewhat toxic, but as long as you wash your hands after contact you'll be fine.

You can clean it off with a variety of substances.  One of the more efficient ones is phosphoric acid, which readily reacts with lead compounds.  You can buy a can of that at the auto parts store for a pretty high price, or just use soda (Coca-Cola, Dr. Pepper, etc.)