Shrimp Poke With Pickled Radishes

Name: Shrimp Poke With Pickled Radishes
Ingredients: 6 red radishes, trimmed, thinly sliced; 3/4 cup white wine vinegar; 3 tablespoons sugar; 1 teaspoon crushed red pepper flakes; 2 teaspoons kosher salt, divided, plus more; 1 1/2 cups short-grain sushi rice; 1 tablespoon dried hijiki (seaweed); 1 1/2 pounds large shrimp, peeled, deveined; 1 medium shallot, finely chopped; 1/4 cup soy sauce; 1 tablespoon plus 2 teaspoons gochujang (Korean hot pepper paste); 4 teaspoons unseasoned rice vinegar; 1 tablespoon fish sauce; 2 teaspoons honey; 1 teaspoon toasted sesame oil; 2 cups baby mizuna or arugula; 2 teaspoons toasted sesame seeds