Place fish into a large Ziplock bag then add lemon juice, oil, garlic salt, chili powder, smoked paprika, cumin, and pepper then gently squish to coat fish and place into the refrigerator to marinate for 30 minutes.
Meanwhile, combine ingredients for the White Sauce in a small bowl then place into the refrigerator. Add ingredients for Avocado and Sweet Corn Slaw into a large bowl, starting with the juice from 1/2 lemon, then toss to combine and taste. Add remaining 1/2 lemon if needed for flavor then set slaw aside.
Heat a large skillet over medium/medium-high heat (6/10 on the heat scale) then spray the bottom generously with extra virgin olive oil or nonstick spray. Add half the mahi mahi then saute for 3-4 minute a side or until cooked through. Transfer to a plate then saute remaining mahi mahi.
Flake fish then place in the center of corn tortillas, top with Avocado Sweet Corn Slaw, and then drizzle with White Sauce and serve.