Bring water to a boil in a medium saucepan. Add quinoa and salt, cover, and reduce to a simmer. Cook 20 minutes, until water is absorbed. Turn off heat and steam with lid on for 5 minutes. Add rice vinegar and fluff with a fork.
Meanwhile, prepare spicy mayo by whisking mayonnaise, lime juice, tamari and cayenne in a small bowl. Season to taste.
Using scissors, slice nori into ½-inch strips. Stack together and cut into ½-inch squares. Stir half of the sliced nori into quinoa once it is cooked.
Heat sesame oil in a large sauté pan over medium-high heat. Add shrimp, and cook about 2 minutes on each side, until pink and no longer opaque. Stir in garlic, ginger and a pinch of salt, and cook for 1 minute more, until fragrant.
To assemble, divide quinoa into 4 bowls. Top with shrimp, a nest of arugula, cucumbers and avocado. Drizzle with mayo, and garnish with remaining nori and sesame seeds.