Heat the milk in a small saucepan over medium-low heat. In another saucepan, melt the butter and whisk in the flour. Cook over low heat for 2 minutes while whisking. While still whisking, add the hot milk and cook for a minute or 2 more, until thickened and smooth. Remove from heat, add the cheeses, salt and pepper. Add the cooked macaroni and stir well. Gently toss with salmon.