Place Quinoa in a medium-sized pan, add the water, and bring to a boil. Reduce the heat to medium-low and simmer, covered, until the liquid is absorbed, 10 to 12 minutes. Remove from heat and fluff it with a fork. Transfer the Quinoa to a large bowl and cool to room temperature.
Sprinkle quinoa with the salt and pepper and fold in 4 tablespoons of the lemon juice and the oil. Gently fold in the cilantro and garlic.
Toss the avocado with remaining tablespoon of lemon juice.
Fold the corn, onion, (chili pepper if using) tomatoes, cucumbers and avocado into the Quinoa. Serve at room temperature.