Combine zucchini, 1/4 cup green onions, and broth in a large saucepan; bring to a boil. Cover, reduce heat, and simmer 5 to 7 minutes or until zucchini is tender. Remove from heat; cool 30 minutes.
While zucchini mixture cools, combine remaining 1/4 cup green onions, cucumber, cilantro, 1 tablespoon lime juice, and 1/4 teaspoon salt in a small bowl; toss well. Cover and chill.
Place remaining 2 tablespoons lime juice, remaining 1/4 teaspoon salt, zucchini mixture, avocado, buttermilk, and cumin in a blender, and process until mixture is smooth. Cover and chill at least 2 hours. Pour soup into bowls, and top with cucumber salsa. Serve chilled.