Mix some raw sugar and coconut flakes and spread them out on a small plate. Rim a collins glass with some lime juice, then dip into the coconut mixture. Carefully fill the glass with ice.
Fill a cocktail shaker with ice and then pour in the juice, tequila, and St. Germain.
Cover and shake vigorously for 30 seconds. Strain into the prepared cocktail glass.
Garnish with lime and grapefruit slices. Serve immediately.