Heat butter and garlic in a skillet over medium heat till the butter begins to bubble. Stir in the shrimp,and cook until they just turn pink about 3 mins. Place 1 shrimp into each mushroom cap,and place into a small baking dish. Spoon the garlic butter into the mushroom caps, and sprinkle each with a pinch of mozzarella.
Bake in a preheated oven til the mushrooms are tender and the cheese is golden and bubbly,10-15 mins.