Rim your glass with sea salt. To do this, pour a generous amount of sea salt onto a small plate. Rub a lime wedge around the rim of the glass, then gently press the rim of the glass into your salt. You may need to wiggle the glass in the salt a little bit to get a decent amount onto the rim.
Add ice to a cocktail shaker, then add in the mezcal, grapefruit soda, grapefruit juice, and simply syrup. Shake vigorously until the shaker feels cold.
Pour into your salted glass, then top off with sparkling water and a grapefruit wedge to garnish.