How about, “That’s not yours.” Or, “That’s no longer clean.” That seems to work in the U.S. Especially now, during the pandemic.
Although there are phrases to indicate that water is no longer drinkable (non-potable, contaminated, etc.), there is really not a universal phrase to indicate that drinkable water should not be drunken by someone else. At least, not in polite company. Generally, we will do things like mark our individual receptacles with writing, symbols, charms, or drink containers of differing colors to indicate that one should drink only from their own beverage. Generally, it is just understood not to drink from/of any item that you can not positively identify as untampered, and yours and yours alone. Especially around strangers.
However, there is a universal phrase in the U.S.A. for a liquid that is to be shared out of the same container. It is the same phrase for sharing a meal item from a central dish for your table (as opposed to a buffet table). It is called ”Family-Style”. It is very typical for alcoholic beverages (or, sometimes even water) in outdoor or rustic environments like camping, farming, sports, and field work to be shared Family-Style. Although, it is typically considered impolite, uncouth, and socially unacceptable to place your mouth or any other body part on the opening of a communal beverage container when there are individual containers to which you can transfer the beverage, even amongst your own family. It is not uncommon to share or allow a family member or really really close friend to taste an item from your own container or utensil.