Chicken chow mein is stir fried chicken with cabbage and celery and other vegetables, in a light sauce. The whole thing is served over crunchy noodles (the "chow mein" in chow mein).
Egg foo yung (or young) is basically an omelette with egg and vegetables, typically shredded cabbage, bean sprouts, and onion, served with a gravy.
Like EricNau said, they're very different from each other.