Peel cucumber so it has alternating green stripes and thinly slice into rounds. Place in a mixing bowl and sprinkle about 1/4 teaspoon salt. Let sit for 15 minutes and squeeze the water out using your hands. Return the cucumber slices to the bowl and set aside.
Soak the wakame in cold water for 5-6 minutes, or according to the directions on the package and drain. Add the wakame to the cucumber bowl along with the sesame seeds.
Mix all the ingredients for the dressing and pour over the cucumber and wakame. Toss using chopsticks and serve, or top with shirasu and serve.
Wakame gets slimy if it’s left in liquid or dressing for too long and because of that this salad is best eaten fresh.