Add the chopped cucumbers, arugula, avocado, Greek yogurt, lemon juice, garlic, salt, and black pepper to a blender.
Blend on high speed until the mixture becomes completely smooth and no large pieces remain.
Taste and adjust salt or lemon juice if needed, then blend again briefly to combine.
Pour the soup into a large bowl or container, cover, and refrigerate for at least two hours until fully chilled.
Stir the soup gently before serving, then divide into bowls and top each with a few arugula leaves and cucumber slices.