Remove 2 tablespoons crab meat for garnish, if desired. Place remaining crab or imitation crab, cream cheese, mayonnaise, mustard, shallots, horseradish, pepper, and salt, if using, in a food processor bowl; cover and process until smooth, scraping sides of bowl. Spoon mixture into a pastry bag fitted with a large star tip.
Using a zester or vegetable peeler, peel the cucumbers, leaving some strips of skin. Cut into 3/4-inch-thick slices. (Or, halve 2 cucumbers lengthwise and slice 3/4-inch thick.) With a melon baller, scoop out the center of each slice to create a cup. Pipe mousse into cucumber cups. Sprinkle each with reserved crab meat, if desired. Cover and chill up to 2 hours before serving. Makes about 40 pieces.