Prepare the salmon and collards:
Whisk the cilantro, lime juice, olive oil, 1 tsp. salt, and 1/2 tsp. pepper in a small bowl. Rub the marinade all over the salmon and refrigerate.
Bring a large pot of well-salted water to a boil. Meanwhile, remove the center stem from each collard leaf. Blanch the greens in the boiling water until they become bright green and pliable, about 3 minutes. Transfer to an ice bath to cool completely. Drain and pat dry with kitchen towels.
Remove the salmon from the refrigerator. Arrange 2 collard leaves, overlapping each other, with short sides facing you, until they are the length of a salmon fillet. Place a fillet about 2 inches from the bottom edge. Roll the fillet completely in the leaves. If necessary, trim the edges of the collards so that the fillet is visible from both sides. Using kitchen twine, tie once around the center of the collard packet to keep it closed. Repeat with the other fillets and collards. Refrigerate until ready to grill.
Make the tomato-mint salsa:
Combine all of the ingredients in a medium bowl and refrigerate until ready to use.
Make the tzatziki:
In a medium bowl, combine the yogurt, garlic, and 1/2 tsp. salt.
Put the grated cucumber in a medium-mesh strainer over a bowl. Sprinkle 1/2 tsp. salt on the cucumber and toss. Let stand for 10 minutes. Squeeze out the excess water with your hands and stir the cucumber into the yogurt along with 1/4 tsp. pepper, and mix well. Cover and refrigerate until cold, about 1 hour. Season to taste with salt and pepper, and drizzle with the olive oil.
Cook the salmon:
Preheat a gas or charcoal grill to medium-heat (350°F to 375°F). Clean and oil the grill. Oil the bottom of the salmon packets. Transfer to the grill, tie side up, and grill, covered, until the greens begin to wilt and darken and an instant-read thermometer inserted in the thickest part of the salmon registers 145°F, 10 to 12 minutes.
Transfer to a cutting board and let rest for a couple of minutes. Remove the twine and serve the salmon, garnished with the mint leaves and accompanied by the tomato-mint salsa and the tzatziki.