Mix corn, avocado, tomatoes, onion, feta, and cucumber.
Add to a large bowl and refrigerate until ready to use.
Add olive oil, sherry vinegar, garlic powder, cilantro, salt, and ground pepper together in a small glass jar with a lid. Shake really well. Taste and adjust seasoning and ratios of oil and vinegar as you desire.
When ready to serve salad, add the dressing and gently toss.