Cutting crosswise, slice 2 to 4 thin rounds from the jalapeño (depending on how spicy you want the drink), plus 1 or 2 extra for garnish. Scoop the 2 to 4 slices into a cocktail shaker, add the simple syrup and muddle with a muddler or the back of a large spoon. Add the tequila or mezcal, grapefruit and lime juices, and 6 to 7 ice cubes and shake for 10 to 15 seconds. Pour the drink into a 10-ounce glass, ice and all, and top with Squirt. (If you prefer, you can strain the mixture into the glass and add fresh ice cubes before topping with the Squirt.) Garnish with 1 or 2 jalapeño rounds.
To make simple syrup, in a small saucepan bring equal parts water and sugar to a boil, stirring or swirling the pan until all of the sugar is dissolved. Remove from the heat and cool. Store in the refrigerator. It will keep for month or more.