Lightly spray the insides of pumpkin and ghost shaped cookie cutters. Press rice into the cutters to shape, then carefully release the rice from the cookie cutters.
Cut out shapes in the seaweed for the ghost eyes and mouth, and for the pumpkin eyes nose and mouth.
Use the same pumpkin cutter to cut out pumpkin shapes in the salmon.
Place the salmon on top of the rice.
Place the seaweed shapes onto the ghosts and pumpkins.
In a small bowl, whisk together the mayo, sesame oil, and sriracha and serve alongside the rice balls for dipping.