Preheat your oven broiler to high and line a baking sheet with foil.
Melt butter in a skillet over medium heat. Add chopped shrimp and cook for 2 to 3 minutes until just pink. Remove from heat and cool slightly.
Mix cream cheese, mayonnaise, Dijon mustard, Old Bay, garlic powder, smoked paprika, and black pepper in a bowl until smooth.
Fold in cooked shrimp, crab meat, and green onions. Taste and adjust salt.
Arrange biscuit halves cut-side up on the baking sheet. Spoon filling evenly over each half.
Top each melt generously with shredded cheese.
Broil for 3 to 4 minutes, watching closely, until cheese is melted and beginning to brown at the edges. Let rest one minute before serving.
