Preheat the oven to 350 degrees F. Bring a large of pot of salted water to boil. Add the macaroni and cook until just tender. Place 2-1/2 cups of the milk into another pot and bring close to boiling. Place the remaining 1/2 cup milk and the flour in a small bowl and whisk until lump-free. Whisk this mixture into the hot milk. Gently simmer, stirring occasionally, until the sauce lightly thickens. (Be careful not to burn.) Mix in 2 cups of the cheese; remove from the heat. When the macaroni is cooked, drain well and place in a bowl. Mix in the cheese sauce, ham, green onions, salt and white pepper; spoon into a 9 x 13-inch casserole. Sprinkle with the remaining cheese. Bake for 30 minutes, or until brown and bubbly.