the cause of the mildew on interior walls

The two sides of my mother's living room are covered with mildew. I will appreciate help in figuring why, it does't seem to fit the typical condition that causes mildew.
Here is some facts about this room... the Bay Area house is located several blocks away from ocean. Weather is foggy and cold most of the time. The house has a box shape, with the living room faces the ocean. The window is open for at least one hour daily. The room is not unheated.
In this room, only the two sides, which are also the two sides of the house, has mildew. The side that faces ocean doesn't appear to have any. These two walls are also covered with fine cracks. From a distance, they look like spider-webs. There are other rooms that share the same sides of the house, but they don't have mildew on it.
Thank you for your help.
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Emily wrote:

It could be due to many reasons. First that room may have more moisture in the air (note, cooler air hold less moisture so the same amount of moisture in cool are results in higher humidity and more problems) If that room (walls) is/are cooler and the air has the same amount of moisture in it, that could be a problem.
It also could be the mold has more to grow on or eat. Old glue from wall paper makes a good environment for mold.
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Emily wrote: /snip/.

Is the mildew just on the outside (interior facing) wall surface? You might want to rule out mildew starting on the inside of the wallboard, possilbly from water intrusion due to say, a slow roof leak. One easy way to check for this is to pull off a light switch or electrical outlet cover.
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