Efflorescence affects masonry materials such as brick, concrete and plaster. It doesn’t damage the underlying surface, but it’s ugly and can cause paint to peel. Wherever it appears, the remedy always starts by stopping the moisture, as the deposits are sure to reappear if you scratch them or scratch and repaint. If the crust is on a ceiling, a roof leak is often the culprit. On walls, leaks around windows and doors are the most common sources. But your note and the photo you sent suggest that the problem areas are in the middle of the wall. For this reason, first go outside and check if you see anything that could be the reason why these areas are harboring moisture. Is there a leaking connection in an overhead gutter? Is the earth stacked against the wall? If so, fix the problem and you should be able to repaint the interior wall without the siding bubbling again.