Bring quinoa, ½ teaspoon salt, and 1 ¼ cups water to a boil in a medium saucepan over high heat. Reduce heat to medium-low, cover, and simmer until quinoa is tender, about 10 minutes. Remove from heat and let stand, covered, for 5 minutes. Fluff with a fork.
Meanwhile, whisk lemon juice and garlic in a small bowl. Gradually whisk in olive oil. Season dressing to taste with salt and pepper.
Spread out quinoa on a large rimmed baking sheet; let cool. Transfer to a large bowl; mix in ¼ cup dressing. DO AHEAD: Can be made 1 day ahead. Cover remaining dressing and quinoa separately; chill.
Add cucumber, tomatoes, herbs, and scallions to bowl with quinoa; toss to coat. Season to taste with salt and pepper. Drizzle remaining dressing over.