Place 1 cup of shredded coconut in a small zip-top bag, add 3–4 drops of green gel food coloring, seal, and shake vigorously for 1–2 minutes until evenly coated. Set aside with the bag slightly open to dry.
Process all 36–39 Oreo cookies in a food processor (or crush in a sealed zip-top bag with a rolling pin) until you have fine, uniform crumbs. Reserve 1 to 1½ cups of crumbs for the topping and transfer the rest to your 9×13-inch baking dish.
Drizzle the 6 tablespoons of melted butter over the crumbs in the dish, toss to combine, then press firmly and evenly into the bottom. Refrigerate for 5–10 minutes to firm up.
Beat the softened cream cheese and 4 tablespoons of softened butter with a hand mixer on medium speed for 2 minutes until smooth, then mix in the powdered sugar and vanilla until fully combined. Gently fold in half the Cool Whip (4 oz) with a rubber spatula and set aside.
In a separate large bowl, whisk together the cold milk, both boxes of instant chocolate pudding mix, for about 2 minutes until thickened. Let it rest for 2–3 minutes to continue setting up.
Fold the pudding mixture into the cream cheese mixture until just combined, then gently fold in the remaining Cool Whip (4 oz) until the filling is smooth, thick, and mousse-like. Do not over-mix.
Pour the filling over the chilled Oreo crust and spread into an even layer, then sprinkle the reserved Oreo crumbs evenly over the top to create the "dirt" and press them down lightly.
Cover the pan tightly with plastic wrap and refrigerate for a minimum of 2 hours (4 hours or overnight is ideal for the best texture and cleanest slices). Do not add candy decorations yet.
Just before serving, scatter the green coconut grass over the top, press in the marshmallow Peeps, and arrange Cadbury mini eggs, pastel M&Ms, jelly beans, or sprinkles across the surface. Slice into 15–18 pieces and serve immediately.