Place shredded zucchini in a colander; sprinkle with the salt and toss lightly. Let stand 30 minutes. Rinse and drain. Squeeze out excess liquid; set aside.
In a saucepan melt margarine or butter. Stir in flour and mustard. Add milk. Cook and stir until bubbly. Remove from heat. Stir in zucchini and cheeses. In a large bowl beat yolks with a fork. Gradually stir in zucchini mixture.
In a mixing bowl beat whites with an electric mixer until stiff peaks form. Fold half of the whites into zucchini mixture. Gently fold remaining whites into zucchini mixture. Spoon into six 6-ounce greased souffle dishes or custard cups.
Bake in a 375 degree F oven for 20 to 25 minutes or until a knife inserted near center comes out clean. Serve immediately. Makes 6 side-dish servings.