Peel cucumber and slice in half lengthwise; remove seeds with a small spoon. Place cucumber in a food processor and pulse about 5 times, until cucumber is minced. Place cucumber into a small glass mixing bowl. Place onion in the food processor, pulsing until it is finely chopped. Add onion to cucumber. Add cream cheese and stir well with a spatula. Add salt, hot sauce, and spirulina powder (for coloring). Cut wheat bread slices and spread a small amount of mayonnaise each one. Spread cheese mixture on another slice and place on top. Garnish with fresh herbs.