In a mixing bowl, stir together the sweetened condensed milk, lemon juice and salt together, set aside.

Remove segments from the grapefruit. Trim off the ends of the grapefruit. Remove the skin of the grapefruit. Place your knife in between the skin and flesh, matching the curve of the skin. Cut in one smooth motion. Cut off just enough to remove all the white pith and expose the flesh.

Place grapefruit on cutting board on its side. Cut toward the center, along a membrane. Then slice along the adjacent membrane until the cuts meet, releasing the segments.

Lightly fold segments into the sweetened milk mixture. Pour mixture into the graham cracker crust. Place in fridge to chill for at least four hours or overnight. Serve chilled, topped with whipped cream if desired.