In a large mixing bowl, combine the mixed frozen fruit and mini marshmallows. Stir gently to combine.
In a separate bowl, mix the whipped topping, sweetened condensed milk, and vanilla extract until well blended.
Pour the whipped topping mixture over the fruit and marshmallows. Gently fold everything together until the fruit is evenly coated.
If using, add the chopped nuts and fold them in carefully.
Transfer the mixture into a 9x9-inch baking dish or a similar-sized container. Spread it evenly.
Cover with plastic wrap or a lid and freeze for at least 4 hours or until solid.
To serve, remove from the freezer and let it sit at room temperature for about 5-10 minutes before cutting into squares.