Place potatoes in a microwave-safe dish; cover tightly with plastic wrap. Cut a 1/2-inch slit in plastic. Microwave at HIGH 6 minutes or until almost tender. Coat grill grate with cooking spray. Add potatoes to grill (skewer if necessary); cook 5 minutes or until tender and well marked, turning after 3 minutes.
Coat fillets with cooking spray; sprinkle evenly with 1/4 teaspoon salt and 1/4 teaspoon pepper. Add fillets to grill; cook 3 to 5 minutes on each side or until desired degree of doneness. Remove fillets from pan; cover, and keep warm.
Coat squash with cooking spray; sprinkle with 1/4 teaspoon salt and 1/4 teaspoon pepper. Add squash to grill; cook 3 minutes or until tender and lightly charred.
Place remaining 1/2 teaspoon salt, remaining 1/2 teaspoon pepper, cilantro, and garlic in a food processor; pulse until finely chopped. Add oil; process 30 seconds or until smooth. Serve cilantro mixture with potatoes, fillets, and squash.