If unopened, fresh mozzarella can last a few days to up to a week past the sell by date. The key here is UNOPENED. Fresh mozzarella cheese generally comes vacuum sealed or in liquid (a brine or water) with a plastic seal on top. These measures help to ensure the cheese stays fresh. Once opened, you have about 3-4 days to use or freeze the remaining before you run the risk of it going rancid.