Trouble Shooting- White Residual on planters. Have you noticed a white residual on your planter? I have too. It is a harmless calcium-carbonate mineral salt called efflorescent. Although it is harmless it can be unsightly. Fortunately it can be washed off. To clean off efflorescence, wash the piece with water repeatedly.The process will be finite there is only a limited amount of soluble salt in the concrete. Going forward I will be using a denser mix and applying a waterproofer to each piece. Please call if you have any questions or issues.