For the simple syrup: Combine the sugar and 1 cup water in a small sauce pot over medium heat. Bring to a simmer and cook, whisking occasionally, until the sugar dissolves, about 5 minutes. Remove from the heat and cool to room temperature. Refrigerate in an airtight container for up to 2 weeks.
For the mojito: Combine the mint, lime wedges, jalapeno and 3/4 ounce of the simple syrup (1 1/2 tablespoons) in a cocktail shaker and muddle. Fill the shaker with the ice, pour in the rum and mango nectar and shake briefly. Pour directly into a highball glass. Top off with the sparkling water and stir briefly. Garnish with a mint sprig and jalapeno slice.