In a microwave-safe bowl, melt the green candy melts by heating them at 50% power for 1 minute. Stir and continue heating in 15-second intervals until fully smooth and glossy, about 2-3 minutes total.

Once melted, carefully add the sweetened condensed milk and the Creme de Menthe to the bowl. Stir vigorously until completely combined.

Cover the mixture with plastic wrap and let it rest for 10-15 minutes, stirring every 5 minutes until thickened.

Prepare a piping bag fitted with a star nozzle and fill it with the cooled fudge mixture. Pipe into cone shapes on a parchment-lined baking sheet.

While the fudge trees are still soft, sprinkle rainbow nonpareils and optional gold star sprinkles over each tree. Let sit at room temperature for about 30 minutes to set.