In a blender add cucumbers, water, mint leaves, and agave nectar and blend until smooth.
With a fine mesh sieve strain the liquid and discard the pulp.
Sprinkle chile-lime powder on a small plate. Using agave nectar, moisten the rims of glasses. Invert the glasses into the chile-lime powder, to coat the rims.
Fill a shaker with ice, cucumber mixture, and tequila. Shake well and serve over ice. Top with a splash of grapefruit soda.
Notes
Garnish with cucumber ribbons and mint, if desired.