Put cream cheese in a bowl and mash with a fork until smooth. Wrap cucumber in cheesecloth, then squeeze out and discard juice. Add cucumber to cream cheese and mix thoroughly. Wrap grated onion in cheesecloth and squeeze juice into cream cheese mixture, then discard onion. Mix mayonnaise and Tabasco sauce into cream cheese mixture. Season to taste with salt, then add 1 drop green food coloring and mix well. Spread mixture on 8 slices of bread, then top each slice with another slice. Trim off crusts, and cut into finger sandwiches.