Toss the slaw mix, red onion, lime juice, sugar and 1/2 teaspoon salt in a large bowl; set aside.
Preheat a grill to medium. Oil the grates. Mix the cayenne with 1/2 teaspoon salt in a small bowl; rub all over the fish. Grill until well marked on the bottom and the fish releases easily from the grill, about 6 minutes. Flip and grill until cooked through, 4 to 6 more minutes. Transfer to a cutting board and slice or flake the fish.
Meanwhile, wrap the tortillas in foil and warm on the grill, about 4 minutes.
Divide the fish among the warm tortillas. Top with the slaw, cilantro, crema and salsa verde. Serve with lime wedges.