Quarter the zucchini lengthwise, then thinly slice with a knife or mandoline. Transfer to a colander set over a bowl and toss with 1/2 teaspoon salt. Let drain 15 minutes.
Combine the zucchini, tomatoes, red onion, jalapeno, lime juice and half of the cilantro in a bowl. Set aside 1 hour.
Preheat a grill to medium high. Mix the cumin and coriander in a small bowl. Brush the fish all over with olive oil, then rub with the spice mixture and season generously with salt and pepper. Grill the fish until it is well marked and releases easily from the grill, about 4 minutes. Flip the fish and continue cooking until firm, 2 to 4 more minutes, depending on the thickness. Transfer to a platter.
Stir the remaining cilantro into the zucchini mixture and season with salt. Spoon over the fish.