Combine nama shoyu and umeboshi vinegar. Submerge arame seaweed to hydrate. Allow to stand for at least 30 minutes or overnight. Arame will soften. Combine cider vinegar, lemon/lime juice, coriander, and pepper in a separate large bowl. Add onion and red pepper; allow to soften.
Meanwhile, chop kale finely (a quarter-inch wide or less). Add to onion and pepper mixture and toss to coat. Add sesame oil. Drain seaweed, reserving liquid. Add seaweed to kale, and one to three teaspoons of the arame soak liquid to taste. Massage slaw by squeezing and pressing between hands and/or bottom of bowl. This breaks down the kale and makes it softer. The dish can be enjoyed immediately, or covered and refrigerated for several hours or overnight to allow flavors to mature.
*Chef’s note: Reserve remaining arame soak liquid to hydrate more seaweed by adding a bit more nama shoyu and umeboshi vinegar. Store it in a sealed glass jar in the refrigerator.