Line an 8-inch square pan or small baking dish with foil or parchment and lightly grease if needed, leaving enough overhang to lift the fudge out later.

Add the white chocolate chips and sweetened condensed milk to the slow cooker. Cover and cook on low for 30 to 45 minutes, stirring every 10 to 15 minutes until the mixture is smooth and fully melted.

Stir in the fresh lime zest until evenly distributed, resulting in a creamy and thick texture speckled with green.

Spread the mixture into the prepared pan, smoothing the top with a spatula. Optionally sprinkle extra lime zest for color.

Refrigerate for 2 to 3 hours, or until firm. Lift from the pan, cut into small squares, and serve.