In a heavy saucepan, combine the marshmallows and 1/4 cup grapefruit juice. Cook and stir over low heat until marshmallows are melted. Cool to room temperature.
Peel the grapefruit and cut into sections (about 2-1/2 cups). Stir the grapefruit, whipped cream and remaining grapefruit juice into marshmallow mixture. Pour into crust. Sprinkle with coconut. Refrigerate for 3 hours or until set.