Bring water and quinoa to a boil in a saucepan. Reduce heat to medium-low, cover, and simmer until quinoa is tender, 15 to 20 minutes. Fluff with a fork.
Meanwhile, set an oven rack about 6 inches from the heat source and preheat the oven's broiler.
Thread mushrooms, halloumi cheese, and tomatoes onto skewers. Cook under the broiler, rotating the skewers a few times, until halloumi beings to turn golden and the vegetables are cooked through, about 5 minutes
Stir pesto and olive oil together in a bowl; add enough oil to reach the desired consistency.
Divide quinoa between 2 plates and place skewers on top. Lightly drizzle with pesto mixture.