In a saucepan, stir together the juice, zest, chamomile flowers, sugar, and water and cook over medium heat. Simmer until the sugar has completely dissolved and the mixture has thickened into a syrup, about 5 minutes. Remove from the heat and allow to cool.
Strain the mixture through a fine-mesh sieve set over a bowl, then transfer to an airtight container. Store in the refrigerator for up to 1 week.
Pour the Rose Salt into a shallow plate or bowl. Wet the rim of a tumbler with a lime wedge and dip the rim in the salt, gently rolling to coat it evenly.
Place the ice in the tumbler and carefully pour in the grapefruit juice, simple syrup, tequila, seltzer water, and orange zest. Stir and garnish with a chamomile flower or a slice of grapefruit.