Bring water and salt to a boil in a heavy medium saucepan. Add grits gradually, stirring, and return to boil. Simmer over low heat, stirring, until thick and porridgelike and grains are tender, about 18 minutes.
Increase heat to moderate and add mozzarella a little at a time, stirring, until melted into grits. (Mixture will be stringy but pliable.) Serve piping hot.
