Preheat the oven to 375°F (190°C). Line a baking sheet with parchment paper.
In a mixing bowl, combine the chopped shrimp, mayonnaise, mozzarella, cheddar, Parmesan, green onions, garlic, salt, and pepper. Stir until well mixed.
Place the ciabatta halves cut-side up on the baking sheet. Brush with melted butter.
Evenly spread the shrimp mixture over both halves of the ciabatta.
Bake for 15–18 minutes, or until the cheese is melted and bubbly.
Slice and serve warm, garnished with parsley if desired.