In a medium bowl, combine the graham cracker crumbs and melted unsalted butter, mixing until the texture resembles wet sand. Spoon this mixture into serving glasses or bowls, pressing it down gently with the back of a spoon to form an even layer.

Chill a mixing bowl and beaters for about 10 minutes before beginning. Add the cold heavy whipping cream and powdered sugar to the chilled bowl. Using an electric mixer, whip the cream on medium-high speed until stiff peaks form, approximately 2-4 minutes.

In a separate bowl, beat the softened cream cheese with an electric mixer until creamy and smooth, about 2 minutes. Gradually incorporate the sweetened condensed milk until fully blended and silky. Then, gently fold in the fresh Key lime juice and zest.

With a gentle hand, fold the whipped cream into the cream cheese mixture in two additions. Use a spatula to carefully incorporate the whipped cream until no white streaks remain.

Using a piping bag or a spoon, carefully layer the airy Key Lime Pie Mousse over the prepared graham cracker crust in the serving glasses. Fill each glass, leaving some space at the top for garnishing later. Chill for at least 30 minutes.

When ready to serve, remove the Key Lime Pie Mousse from the refrigerator. Garnish with thin lime slices, a sprinkle of graham cracker crumble, or extra lime zest.