Add thinly sliced cucumbers to a medium bowl. Sprinkle 1/2 tsp salt and mix with hands, scrunching the cucumbers a little. Set aside.
In a large bowl, place dried wakame. Add 2 cups cold water and soak for 10 minutes. The wakame will swell and rehydrate until soft enough to eat.
After the wakame rehydrates, drain and gently squeeze out excess water. Do not rinse the wakame! Add cucumbers, squeezing out excess liquid. Add fresno chili pepper, if using. Add rice vinegar, sugar, and sesame oil. Gently mix together until well combined.
Transfer to a serving bowl, leaving behind any excess liquid. Serve room temperature or chilled. The salad should last 4-5 days, refrigerated.