Step 1: In a large bowl, whisk together the instant pistachio pudding mix and cold milk for 2 minutes until slightly thickened. Set aside.
Step 2: In a separate bowl, beat the softened cream cheese and granulated sugar with an electric mixer until smooth and creamy, about 2-3 minutes.
Step 3: Gently fold the thawed whipped topping into the cream cheese mixture until fully combined.
Step 4: Add the pudding mixture to the cream cheese and whipped topping mixture. Fold gently until just combined.
Step 5: If desired (especially if using vanilla/cheesecake pudding), add a few drops of green food coloring until you achieve your desired vibrant Grinch green color.
Step 6: Fold in the mini marshmallows.
Step 7: Transfer the Grinch Fluff to a serving bowl. Cover tightly with plastic wrap and refrigerate for at least 4 hours, or preferably overnight, to allow it to set and flavors to meld.
Step 8: Before serving, garnish with red sprinkles or mini candy hearts to represent the Grinch's heart, if using.