Place the rinsed quinoa, water, and 1/4 teaspoon of the salt in a medium saucepan. Bring to a boil over medium-high heat.
Reduce the heat to low, cover, and simmer until the water is absorbed and the quinoa is tender, about 14 to 15 minutes. Remove from the heat and let stand, covered, for 5 minutes.
Fluff the quinoa with a fork. Spread it on a large plate or rimmed baking sheet to cool quickly, about 10 minutes, stirring once or twice.
In a large bowl, whisk the lime juice, olive oil, optional ají amarillo paste, garlic, cumin, black pepper, and remaining 1/4 teaspoon salt until combined.
Add the cooled quinoa, cucumber, tomatoes, bell pepper, corn, red onion, and cilantro. Toss gently until evenly coated.
Fold in the avocado just before serving. Taste and add another pinch of salt, lime juice, or ají amarillo paste if desired. Serve chilled or at cool room temperature.