In a saucepot over medium-high heat, add 3 cups water and a pinch of salt. Bring the water to a boil, and then stir in the grits and milk. Raise the heat to bring to a boil again, and then lower to a simmer and cook for about 4 minutes. When the grits begin to thicken but there is still plenty of moisture left, add the mozzarella and Cheddar along with a few grinds of pepper. Stir to melt the cheese, taste and season with salt, if needed. Serve warm.