Season salmon all over with salt and pepper. In a large bowl, mix together cream cheese, mozzarella, spinach, garlic powder, and red pepper flakes.
Using a paring knife, slice a slit in each salmon to create a pocket. Stuff pockets with cream cheese mixture.
In a large skillet over medium heat, heat oil. Add salmon skin side down and cook until seared, about 6 minutes, then flip salmon. Add butter and squeeze lemon juice all over. Cook until the skin is crispy, another 6 minutes. Serve warm.