Prepare the pesto: Preheat oven to 400°F. Process kelp, water, and garlic in a food processor until smooth. Add walnuts, lemon juice, and oil; process until well blended. Add additional water, if necessary, to achieve desired consistency. Stir in salt.
Prepare the pizza: Shape dough into a thin round, and place on a dry baking sheet. Spread with pesto all the way to the edges. (This will ensure that the crust stays flat when baked, rather than puffing up like pizza crust.) Top evenly with asparagus.
Bake in preheated oven until crust is browned, 20 to 25 minutes. Top hot flatbread with Parmesan and radishes.