White stain is one of the most troublesome cosmetic defects on masonry. The French have a word for it is efflorescence, which means “to flower out.”
This is a reaction to when the masonry surface meets moisture. When the water dries, it can leave behind salt, and the deposits are an eyesore.
Although the efflorescence effect is naturally occurring, it doesn’t mean you have to live with it. There are ways to reduce its likelihood of happening in the first place and address it when it manifests itself.