In a medium saucepan, combine 2 1/2 cups of coconut milk with the sugar and iodized salt. Heat on low until it begins to steam but not boil, stirring occasionally.
In a separate bowl, whisk together the remaining 1/2 cup coconut milk with the cornstarch until smooth.
Slowly pour the cornstarch mixture into the heated coconut milk mixture, whisking constantly to combine.
Continue to cook over low heat, stirring constantly, until the mixture thickens enough to coat the back of a spoon.
Remove from heat and whisk in the prepared instant mashed potatoes until fully combined. Allow the mixture to cool to room temperature, then refrigerate for at least 4 hours or overnight.
While the mixture is chilling, create a marbled chocolate mixture by melting the dairy-free dark chocolate chips in a double boiler or microwave. Add a few drops of green and yellow food coloring to the melted chocolate and swirl together to create a marbled effect. Spread the marbled chocolate onto a parchment-lined baking sheet and let it set in the refrigerator.
Churn the chilled potato ice cream mixture in an ice cream maker according to the manufacturer's instructions.
Break the marbled chocolate into irregular shards. As you transfer the churned ice cream into a lidded container, fold in the marbled chocolate shards, creating layers as you go.
Freeze until firm, at least 4 hours.