Rim the glass with salt if desired by rubbing a lime wedge around the rim and dipping in coarse salt.
Fill a cocktail shaker with ice. Add the mezcal, grapefruit juice, lime juice, and agave syrup.
Shake well for about 15 seconds until chilled.
Strain into a glass filled with ice and top with sparkling water.
Garnish with a grapefruit wedge and serve immediately.