Make tombstones. Melt 9 oz. black and 18 oz. white Candy Melts candy, separately, in microwave-safe bowls according to package directions. Mix together to make gray melted candy. Use melted gray candy in cut disposable decorating bag to fill mold. Tap mold gently to smooth and remove air bubbles. Chill until firm, about 10 to 15 minutes. Gently flex mold to unmold candy tombstones.
Make candy treats. Melt light cocoa Candy Melts candy in microwave-safe bowl according to package directions. Mix chopped nuts and marshmallows in melted light cocoa candy until coated. Form mixture into rectangles, each 3 1/2 in. x 2 1/4 in., on parchment paper-covered cookie sheet. Place candy tombstone at top of each treat mixture. Chill until firm, about 10 to 15 minutes.