Place the chopped pear into a bowl along with the grapefruit juice and let sit for 30 minutes.
Meanwhile, with an apple corer, cut out round holes into the tops of each watermelon cube.
In another bowl, add the coconut cream, mint, pistachios and sugar and a tsp of the grapefruit juice and mix well to combine. Place back into the fridge.
Place the watermelon cubes onto a serving tray. Fill each watermelon with the drained pear. With a teaspoon, pour some of the cream over top and scatter with mint leaves. Serve immediately.