Preheat oven to 350°F. Mix shrimp, tomato, green onion, olive oil, egg white, Old Bay Seasoning and garlic powder in medium bowl until well blended.
Slice bread into 24 (1/2-inch thick) slices. Place on baking sheet; broil until lightly toasted on both sides. Spread lightly with mayonnaise. Spoon shrimp mixture evenly over bread slices.
Bake 8 to 10 minutes or until heated through and lightly browned.