Place clams in a bowl with lightly salted water to cover. Place kelp in a separate bowl with 1 1/3 cups water. Let clams and kelp soak for 30 minutes. Drain, reserving only the kelp water.
Cut off and discard the bottom 1 1/2 inches (roots) of the enoki mushrooms. Wash the mushrooms.
Heat olive oil, chili powder, and garlic in a pot over medium heat. Stir in the clams, kelp water, and tofu. Bring to a boil. Add enoki mushrooms, oyster mushrooms, soy sauce, and salt; bring to a boil. Add spring onion, black pepper, and sesame oil. Cook until flavors are intensified, 3 to 6 minutes more.