Place fish in a shallow dish and squeeze the juice from one lime over the fish. In a small bowl, whisk together garlic powder, cumin, cayenne and sea salt. Sprinkle over fish and gently rub the seasoning in. Allow the fish to marinate for about 10 minutes.
Meanwhile, in a large bowl combine cabbage, onion, cilantro, juice from the lime, mustard, honey, salt and pepper. Toss to combine. Set aside. Taste slaw and season with additional salt and pepper, if desired.
Make avocado crema by combining avocado, yogurt, cilantro, lime juice, salt and water in a blender or food processor and blend until smooth and combined. Set aside.
Heat grill to medium-high and lightly oil grill-grate with cooking spray. Arrange tortillas in a stack and wrap with aluminum foil. Grill fish until cooked through, 3 to 4 minutes per side. Place tortillas on cooler part of grill (we used the top rack) until warm, 5 to 8 minutes, turning halfway. Alternatively, you can warm the tortillas in the oven and cook the fish on the stovetop over medium-high heat with a little olive oil for about 3 minutes each side.
Break fish into large pieces. Divide fish among tortillas and top with slaw and avocado crema. Serve each taco with a small lime wedge for squeezing over the taco before eating.