Add all the watermelon and the liquid Stevia to a blender and blend well. Make sure there are no black pits. If there are black pits, remove them before blending them. White pits are fine and edible.
Blend really well. Blend until all pieces are liquid. Then put to the side.
Slice the plum into ⅛-1/4 – inch thick pieces and then halve them. What you want to end up with is as many half-moons as you can. Once you are unable to get half moons, just slice, chop or dice the rest.
Break out your popsicle mold. Before you add the watermelon mixture, add the plum first. Start with the half-moons. Add one half-moon to each mold and push it down. Then add a few other pieces. Do not push down the other random pieces, let those fall where they may. You should have enough plum pieces for all 12 popsicles. Remember, plum is just a sidekick.
Re-Blend the watermelon mixture again so it doesn’t separate. You will notice like oil and water that the mixture does separate, the juice will fall to the bottom, while pulp rises to the top. So make sure you re-blend each time you pour into the measuring cup.
Pour the mixture into a measuring cup and pour into each mold filling it to the top. Do this for all 12 popsicle molds. Tap the molds on the table to make sure all air bubbles are released and the mixture is able to bypass the plum.
Cover the mold with the lid, add popsicle sticks and freeze for 6 hours or overnight.
The next day, run the mold under hot water until you are able to remove the lid. Then continue to run it under hot water until each popsicle is able to be pulled out of the mold. Store in the freezer until ready to serve. It can be stored for up to 6 months.