1. Put the dashi, water, jalapeno, potato, celery, and shallot into a medium pot and bring to a boil over high heat. Reduce the heat to medium-low and simmer until the potatoes are tender, 10-15 minutes.
2. Add the white and brown miso, stirring until dissolved, then add clams, shrimp, and salmon and gently simmer, covered, until clams open and shrimp and salmon are cooked through, 3-5 minutes. Discard any clams that do not open.
3. Divide the soup between four soup bowls and serve hot.
