Wrap ricotta tightly in cheesecloth; place in a colander over a large bowl. Place a heavy object (such as a bowl or canned good) on top; let drain in the refrigerator at least 3 hours (or up to overnight).
Preheat oven to 350 degrees. Coat a 10-inch springform pan with cooking spray. In a food processor, combine drained ricotta, spinach, basil, nutmeg, 1 teaspoon salt, and 1/4 teaspoon pepper. Process until smooth, about 1 minute, scraping sides of bowl as needed. Add whole eggs and egg white; process for 5 seconds. Pour mixture into prepared pan; bake until set and just brown around the edges, 30 to 35 minutes. Let cool completely on a wire rack, then refrigerate, covered, until cold, preferably overnight. To unmold, run a paring knife around edge before removing side of pan.
Cut into quarters. Using a wide spatula, transfer to a cutting board coated with cooking spray. Cut out shapes with a skull-shaped cookie cutter coated with spray (if cutter doesn't come with eyes and a mouth, use aspic cutters to make features). Using a cotton swab, gently press skulls out directly onto crackers (leave eye and nose shapes in cutter).
Be sure spinach is completely dry. Frozen spinach can be substituted for fresh: cook, drain, and press excess moisture from a colander.