In a microwave-safe bowl, combine the green candy melts. Heat in the microwave for 1 minute, then stir. Continue heating in 15-second intervals until fully melted and smooth, about 2-3 minutes total.

Stir in the sweetened condensed milk and Creme de Menthe until fully combined. Allow to cool slightly, stirring every 5 minutes until it thickens to a frosting-like consistency, around 10-15 minutes.

Prepare a piping bag with a star tip and fill it with the fudge mixture. Pipe into tree shapes on a parchment-lined baking sheet, creating a swirl effect for a height of about 2-3 inches.

While the fudge is still soft, place a gold star sprinkle on top of each tree and generously sprinkle rainbow nonpareils over the fudge.

Allow the fudge trees to set at room temperature for about 30-60 minutes, or refrigerate for 15-20 minutes to speed up the process.

Store in an airtight container at room temperature for up to 2 weeks, or wrap and freeze for up to 2 months.