In a small saucepan, combine the coconut milk, milk, sugar, lime zest and agar flakes. Stir to combine with a rubber spatula; let sit 10 minutes to let the agar soften. Over medium heat, slowly bring the mixture to a simmer, stirring often. Cook at a gentle simmer (adjusting the heat as necessary), stirring often, for 4-5 minutes or until the agar is mostly dissolved and the mixture coats the back of a spoon. Pour the mixture through a fine-mesh strainer into a large measuring cup or bowl (something with a spout will make it easy to pour into the ramekins), pressing the agar through (be sure to scrape the bottom of the strainer). Pour the liquid into the prepared ramekins. Chill, uncovered, until set, about 1 hour. Do Ahead: The panna cotta can be made up to 2 days in advance. After the panna cotta has set, cover the ramekins.