For the passionfruit "yolks": Combine the distilled water with the sodium alginate in a flat-bottomed container. Using an immersion blender, mix to combine until no lumps are floating on the surface, 5 to 7 minutes. Cover with plastic wrap and allow it to rest for 20 to 30 minutes. (You want all the air bubbles created during blending to escape before forming the spheres.)
Meanwhile, in a blender, combine the passionfruit puree with the calcium lactate until fully incorporated. Place in a squeeze bottle.
Set aside two small mixing bowls filled with water for rinsing the spheres. Take a small 1- to 2-ounce ladle and just barely fill the bottom of it with 1 teaspoon of the sodium alginate mixture to make it "nonstick". Squeeze about 1/2 ounce passionfruit mixture into the ladle and slowly submerge into the sodium alginate mixture. Once submerged, swirl the passionfruit "yolk" in a circular motion to float it off of the ladle. Keep it in the solution for 3 minutes. (You can do multiple spheres at the same time, but make sure they don't touch each other while in the solution because they will stick together.) Carefully remove the spheres using a slotted spoon and place them in the mixing bowl filled with water to rinse the excess sodium alginate. Serve immediately, or store in mango juice for up to 2 days.
For the salmon poke: Place the salmon in a large bowl. Cut the serrano in half and remove the seeds, stem and membrane. Dice into small pieces and mix into the salmon.
For the dressing: Mix together the ponzu, seaweed salad, sesame oil, ginger and radishes in a bowl. Toss the salmon with the dressing.
For the avocado: Add the avocados, lime juice, salt and sesame seeds to a bowl. Smash the avocado and mix well.
Place the tostada shells on a serving platter. Spread an even layer of avocado over the shells. Add a few spoonfuls of salmon poke. Finish each tostada with a passionfruit "yolk". Serve.