Green copper chloride stains are caused by a reaction between copper pipes and the very small amount of natural salt that is in most domestic water. A simple remedy is to cover the stain area with vinegar and then sprinkle it with salt. Let the solution stand until the green stain is gone.

Another method is to apply household ammonia to the stain. After approximately 15 minutes, rinse it off and then apply straight vinegar. Repeat as needed.

Note, many newer homes and buildings have plastic pipes, not copper.