Preheat an outdoor grill for medium heat and lightly oil the grate.

Place bacon in a large skillet and cook over medium-high heat, turning occasionally, until evenly browned, about 10 minutes. Drain bacon slices on paper towels and finely chop.

Mix cooked bacon, ricotta cheese, mozzarella cheese, green onions, egg, garlic powder, and pepper together in a large bowl. Load mushroom caps evenly with mixture.

Cook mushrooms on the preheated grill until cheese is melted and slightly brown, 8 to 10 minutes.