How do I get a water stain off a painted wall?

What is this wall stain, and how do I get rid of it?

• I've got this white stain on my wall. It was coming through the wallpaper, so I removed the wallpaper and painted over it. Now it's coming through the paint. It's a white stain higher up, and seems to have just darkened the paint farther down. I thought it might be mold, but my friend who works in a paint store where they sell mold-resistant paint says he thinks it's not mold and is actually salt leaching in through the wall. It's a plaster/brick wall. What is this stain, and what can I do with it? Thanks. Here is a picture of the stain: http://s15.postimage.org/8923574ga/Picture_001.jpg

• Answer:

  It is caused by water from above that is bringing the gypsum from the wall and through the paint. Gypsum is a salt and is present in most wall material. Fix the water problem first, dry the wall thoroughly and then seal and pain it.