1. In a blender or food processor, in batches if necessary, whirl cucumbers, yogurt, sour cream, green onions, dill, garlic, olive oil, lemon juice, and mustard until smooth. Add salt and pepper to taste. Transfer to a bowl, cover, and chill until cold, at least 2 hours and up to 6 hours.
2. Ladle chilled soup evenly into bowls. Top each serving with about 1/4 cup crab.