Combine the mousse ingredients in the bowl of food processor and blend until the mixture is smooth. Chill at least 30 minutes. Cut each cucumber crosswise into approximately 20 slices, each slightly less than 1/4" thick.
Cucumbers can also be peeled, striated with fork tines or cut into decorative shapes with biscuit cutters. Assemble hor d''oeuvres no more than an hour before serving by softening the mousse with a wooden spoon and putting it in a pastry bag with a leaf tip. Pipe mousse on top of each cucumber slice and garnish with a small watercress leaf.
NOTE: If you prefer, you can slice the cucumbers a bit thicker than 1/4" and make a small hollow for the filling in the center of the slice with a melon-ball scoop.