In a blender combine avocado, yogurt, lime juice, garlic, and salt. Blend until very smooth, adding enough cool water to create a thick sauce-like consistency. Set aside.
Bring a large saucepan of well-salted water to a boil, and cook rotini according to package directions. Drain well, and rinse under cold running water.
In a large bowl toss together rotini, black beans, corn, avocado, tomatoes, cilantro, and green onion. Add dressing to taste and toss to coat. Taste and add more salt or lime juice if needed.
Store refrigerated for up to 5 days.