Preheat oven to 350°F. Cut the croissants through the equator on a slight diagonal, leaving one side still attached. Fill each croissant with 2 ham slices and 1 cheese slice. Place 2 cashews in each sandwich so they poke out, pointing downwards like fangs. Transfer to a baking sheet lined with parchment paper; bake until cheese is melted, 10 to 12 minutes.
To decorate, use a paring knife to cut out circles from the cucumber for eyes. Using scissors, cut the seaweed snacks into long, thin triangles for eyebrows. Place on croissants.
Tweak these treats for kids with allergies. Slice mini marshmallows into triangles to create vampire fangs.