Preheat the oven to 350 degrees F. Coat two 8-inch round cake pans with cooking spray. Prepare the cake mix as directed; divide the batter between the pans and bake as directed. Let cool 15 minutes, then remove to a rack to cool completely.
Trim the domed tops of the cakes with a long serrated knife to make them level. Spread frosting on one cake layer, then invert the other layer on top. Spread a thin layer of frosting around the outside of the cake and on top. Reserve 1/4 cup frosting for the wasabi.
Melt the candy melts in the microwave, stirring every 30 seconds, until smooth. Add the cereal and stir.
Mark a 3 1/2-inch circle in the middle of the cake with a paring knife. Arrange the dried mango in half of the circle, trimming as needed. Toss the apple with the lemon juice and arrange cut-side up next to the dried mango, pushing it into the cake.
Wrap a 5-by-26-inch strip of parchment around the cake so that it is slightly taller than the cake. Spread the cereal mixture around the apple and dried mango (the parchment will create a wall for the cereal mixture). Refrigerate the cake.
To make the seaweed, microwave the marshmallows and 1/4 cup water in a bowl in 30-second intervals, stirring, until smooth. Transfer to a stand mixer. Beat in the confectioners' sugar with the paddle attachment, then gradually beat in the cocoa powder until smooth. Beat in the butter.
Dust a piece of parchment with cocoa powder and roll the chocolate mixture into a 5-by-26-inch strip. Trim the long sides to make them straight.
Remove the parchment from the cake. Carefully wrap the chocolate strip around the cake, making sure the top of the strip aligns with the top of the cereal mixture. Trim the bottom edge so it's flush with the bottom of the cake. If the strip tears, just press it back together. Transfer the cake to a board.
For the wasabi, tint the reserved frosting with green food coloring and freeze until stiff, 15 minutes; scoop onto the board. For the pickled ginger, flatten the Starburst candies, shape into curvy pieces and place on the board.