In my basement, there is a powdery, bright white substance between some of the bricks. It looks like it was something applied on top of the mortar joint. I scraped a couple places where it was falling out, and about 1/8 inch below is hard mortar.
If I touch it lightly, it falls apart and indeed has been falling off the wall and onto the floor by itself. Basically it's expanding slowly and pushing the coat of paint off the wall.
This wall is below grade. I had a friend stop by who happens to be a mason, and he said it wasn't anything to worry about. But what is the white stuff?