Place plums in a large bowl. In another bowl, combine the sugar, tapioca, almond extract and salt. Cut in butter until crumbly; gently stir into plums. Let stand for 15 minutes.
Transfer to a greased 2-1/2-qt. baking dish. Roll out pastry to 1/8-in. thickness; make a lattice crust. Place over filling. Trim edges. Beat egg white until foamy; brush over pastry. Bake at 425° for 40-45 minutes or until golden brown.