To a blender*, add frozen banana, cucumber, coconut milk, spinach, spirulina, and hemp seed (optional). Blend until creamy and smooth, scraping down sides (or using smoothie wand) as needed.
If too thick, thin with water or more coconut milk. If too thin, thicken with ice or more frozen banana. Taste and adjust flavor as needed, adding more banana for sweetness, cucumber for freshness, or greens or spirulina for green color. See note about spirulina (brands + how much to add).
Serve immediately as is or in my preferred: top with blueberries, granola, and more hemp seeds. Best when fresh, though leftovers will keep covered in the refrigerator up to 2 days or the freezer for 1 week. Thaw to enjoy from freezer.