Lightly oil the grill rack. Grill corn and pepper, covered, over medium heat 10-12 minutes or until tender, turning occasionally. Cool slightly.
Meanwhile, sprinkle fish with salt. Grill fish and squash, covered, over medium heat 7-9 minutes or until fish just begins to flake easily with a fork and squash is tender, turning once.
Cut corn from cob and place in a bowl. Chop pepper and squash; add to corn. Stir in tomato, onion, cilantro, lime zest and lime juice. Serve fish in taco shells; top with corn mixture and avocado slices.