In a large bowl combine pistachio pudding mix and pineapple and stir to combine. Stir in cool whip until smooth. Add 1 cup marshmallows and gently stir to combine. Spread mixture into a large casserole dish (you can also serve this in a large bowl or trifle dish). Scatter remaining marshmallows and pecans over the top of the mixture. Cover and chill until ready to serve.