Cook macaroni according to package directions; drain. In a bowl stir milk into soup. Add chicken, half the cheese, the mushrooms, pimiento, and cooked macaroni; mix well. Turn mixture into a 2-quart casserole. Bake, covered, at 350 degrees for 50 minutes. Uncover and stir. Top with the remaining cheese; bake till cheese melts, 2 to 3 minutes longer.