Lightly spray a 6-quart slow cooker with nonstick cooking spray.
Place the grits in a medium mixing bowl and cover with water to rinse any debris. Stir gently and skim off any debris that floats to the top. Drain the grits and discard the water.
Add the grits, 3½ cups of water, the milk, butter, and salt to the slow cooker. Turn the slow cooker to the low setting, replace the lid, and allow to cook for 6 hours. Stir the grits well before serving.