Preheat to 375°F (190°C). Line a baking sheet with foil and lightly grease.
Mix shrimp, crab, cream cheese, mozzarella, mayo, mustard, seasonings, lemon juice, and parsley until smooth.
Slice a pocket into each salmon fillet, about ⅔ through, without cutting all the way.
Fill each pocket with the seafood mixture, packing gently.
Place fillets on the baking sheet and brush tops with olive oil.
Bake for 18–20 minutes until salmon flakes and filling is hot. Broil 1–2 minutes for a golden top.
Rest briefly before serving. Add lemon wedges or salad on the side.
Tip: If filling spills out, reduce the amount or press it in more firmly.