Do you put plastic behind a retaining wall??

im building a retaining wall 39cm high and noticed my neighbours wall similar to what i am doing has got this white stuff appearing on the blocks, looks like salt damp. i dont know if they have anything between the blocks and the dirt, but what should i do?? should i put the retaining wall up and place plastic along the back of the wall before filling it with dirt to stop this salt damp from happening to my wall?? water will still drain out the bottom but wont go through the blocks.

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