Drain crushed pineapple very thoroughly in a colander, pressing out as much liquid as possible. Excess liquid will make your fluff runny.
Chop fresh strawberries into small pieces, setting aside a few whole strawberries for garnish.
In a large mixing bowl, gently fold together Cool Whip, strawberry pie filling, and sweetened condensed milk until just combined.
Add drained pineapple, mini marshmallows, chopped pecans, and chopped strawberries. Fold gently until everything is evenly distributed.
Transfer to serving bowl or keep in mixing bowl. Cover with plastic wrap and refrigerate for at least 1-2 hours (overnight is even better).
Just before serving, garnish with reserved whole strawberries.
Serve chilled and watch it disappear.