Pour rice into a large colander. Rinse under cold running water, stirring with your fingers, until water runs clear, about 5 minutes.
Set colander over a large bowl. Leave rice to drain, about 20 minutes.
Cut sides away from each cucumber round; discard seeded center. Cut sides into thin strips for the pumpkin 'stems'. Place cucumber strips in a bowl.
Cut 1/2 the nori strips into triangles to make the 'eyes.' Cut remaining nori strips into jagged 'mouths.' Place eyes and mouths in separate bowls.
Combine rice, 3 1/2 cups water, and 1 teaspoon salt in a large pot over medium heat. Bring to a boil, stirring occasionally. Reduce heat to low and cover; cook until water is absorbed, about 10 minutes. Turn off heat but keep pot on the burner. Let rice stand, covered, until tender, about 10 minutes.
Remove rice from heat; stir in gochujang and turmeric until rice is uniformly orange. Cool rice until easily handled, about 10 minutes.
Stir 1 cup warm water and salt together in a shallow bowl. Line a platter with plastic wrap.
Scoop warm rice with a 1 1/2-tablespoon cookie scoop and drop onto the plastic wrap. Dip your hands in the warm salted water and roll rice into balls. Stick on 2 eyes and 1 mouth. Press 1 cucumber strip into the top. Transfer finished onigiri to another serving platter.
