Preheat grill to medium-high heat (400-450°F). Place shucked corn ears directly on grill grates. Cook 10-12 minutes, turning every few minutes until kernels develop golden-brown char marks.
Remove corn from grill and let cool 5 minutes. Stand each ear upright in large bowl and slice downward with sharp knife to remove kernels.
Dice cucumber into 1/2-inch pieces and add to mixing bowl. Add finely chopped red onion and cilantro.
In small bowl, whisk together Greek yogurt, mayonnaise, minced garlic, lime juice, chili powder, salt, and pepper until smooth and well combined.
Add cooled corn kernels to bowl with cucumber mixture. Pour dressing over vegetables and toss gently until everything is evenly coated.
Cover bowl and refrigerate for at least 30 minutes to allow flavors to meld. Salad tastes best when chilled.
Before serving, stir salad and taste for seasoning. Add more lime juice or salt if needed. Garnish with extra cilantro and optional cotija cheese.
Serve chilled as side dish or taco topping.