Cheese – Storing Unopened Mozzarella

I'm not sure when that about.com article was written, but if it was posted less than about about 8 months ago, it may have plagarized The Culinarian: A Kitchen Desk Reference by Barbara Ann Kipfer. In that, Kipfer provides "Hints" after the description of what feta is (p. 224-225; you can view it on Google Books):

Consumers who dislike feta's salty taste may soak the cheese in fresh water to leach out some of the salt...Feta cheese is best when eaten fresh, so always check the date. If you will not be consuming it immediately, store feta cheese in a brine or milk bath in the refrigerator. The milk bath will reduce the saltiness and help keep the cheese moist and mild in flavor. Properly stored in brine or milk and refrigerated, feta cheese will last for up to three months. Feta cheese is not a candidate for freezing. Barrel-aged feta sold straight from the barrel may be wrapped in lightweight paper, then wrapped in a plastic bag or plastic wrap and stored in the refrigerator. Keep the feta in paper, even when the paper gets soggy from the cheese moisture.

Emphasis is mine to show you that the bit about the milk bath matches about.com's article word for word.

Since The Culinarian was published by Wiley & Sons, a reputable publisher that puts out a lot of textbooks and academic and trade journals, I would assume that it was properly fact-checked. Given that Kipfer talks about salt leaching out into regular water and the milk bath reducing the saltiness of the cheese, I would guess that there's a strong chance enough salt leaches into the milk bath to act as a preservative.

Take all this with a grain of salt, though (no pun intended); I've never tried keeping feta in a milk bath myself.

Mozzarella comes in different variations... There are a few things to look and smell for to determine it they are "off". Aged or drier varieties may have visible mold on the exterior. Pre-grated mozzarella can also exhibit visible mold as well as a sour or yeasty smell. Brined varieties may have a musty, sour, or smelly-foot odor or an excessively cloudy brine.

If you see visible mold on a drier variety, you can just cut it off and use the remaining cheese as normal as long as it doesn't smell. If you plan on cooking or melting the cheese, it would likely be safe to eat, but may not taste normal.

Moist cheeses like mozzarella can harbor Listeria, E coli, brucella, and salmonella so if you are in doubt and not interested in a bout of food poisoning, it's best just to throw it out.

To keep brined varieties fresh longer keep it refrigerated and replace the brine daily, the replacement brine should taste salty like the ocean. Even so, it should really be eaten within 2-3 days.