I have a white powder on my brick. What is this, and what do I do about this?

This white powder is called efflorescence and is caused by water in the wall dissolving mineral salts in the brick or mortar. When this leaches to the surface, the water evaporates, leaving a whitish mineral-salt residue. Most efflorescence is water soluble, so it can usually be removed by washing with plain water when the weather and the wall are warm and dry. This helps keep the cleaning water from causing additional efflorescence.

The only long-term cure for this condition is to stop water from entering the wall by ensuring that any flashing or caulking is correctly installed and defect-free.