Step 1

In a blender, puree all of the ingredients except the xanthan gum on high speed until smooth and bright green, about 30 seconds. Pour into a bowl. Slowly sprinkle the xanthan gum over while quickly whisking, until slightly thickened and foamy. Let rest for 15 minutes, then skim any foam. Pour 1 1/2 tablespoons of the mojito layer into eight 3-ounce popsicle molds or paper cups. Freeze until almost firm, 1 hour. Poke a popsicle stick into the center of each.

Step 2

In another bowl, combine all of the ingredients except the xanthan gum. Slowly sprinkle the xanthan gum over while quickly whisking, until slightly thickened. Let rest, then skim.

Step 3

Carefully add 1 1/2 tablespoons of the watermelon layer to the mojito layer in the molds. Freeze until almost firm, 1 hour. Pour the remaining mojito mixture on top and freeze until almost firm, 1 hour. Top with the remaining watermelon layer and freeze until firm, 2 hours. Unmold and serve.