Peel decorative stripes on the cucumber; cut into 2‑inch sections. Hollow with a melon baller to form cups and chill in ice water.
Halve cleaned shrimp lengthwise. Toss with lemon juice, red pepper julienne, sea salt, and cracked pepper. Chill 30 minutes.
Drain cucumber cups and pat dry. Fill with marinated shrimp; drizzle with olive oil. Garnish with scallion chiffonade.