1/2 cup water
2 envelopes unflavored powdered gelatin (about 2 tablespoons)
2/3 cup strained freshly squeezed lime juice (from about 8 medium limes)
2/3 cup granulated sugar
1 cup strained freshly squeezed orange juice (from about 4 large Valencia oranges)
1 cup passion fruit juice
Green food coloring
Pour the water into a small, wide bowl and sprinkle the gelatin evenly across the surface of the water. Set aside to bloom and absorb the water.
Heat the lime juice and sugar in a small (1-quart) saucepan over medium heat, stirring frequently until the sugar dissolves and the juice comes to a low simmer. Stir in the bloomed gelatin and cook until fully dissolved in the juice, about 1 minute more.
Remove from the heat and stir in the orange and passion fruit juices. Add green food coloring 1 drop at a time to reach your desired level of Slimer green.
Pour into an 8- or 9-inch square baking dish. Refrigerate until set, at least 3 hours. Scoop and serve.