Preheat the oven to 375 degrees and line a baking sheet with parchment paper.
Wipe the mushrooms clean with a damp cloth, then remove and finely chop the stems.
In a mixing bowl, combine the cream cheese, mozzarella, minced garlic, chopped parsley, salt, pepper, and chopped mushroom stems until evenly mixed.
Spoon the cheese mixture into each mushroom cap, filling them generously and smoothing the tops.
Place the filled mushrooms on the baking sheet and bake for 20 minutes until the cheese is melted and lightly golden.
Remove from the oven and press one black olive slice into the top of each mushroom to form eyes.