The answer to 'retaining wall turning brown' is that a retaining wall turning brown is a common issue caused by a few different factors. The most common causes are water damage, mold growth, or natural weathering and aging of the materials used to build the wall.
If the wall is turning brown due to water damage, it may be a sign of improper drainage or poor construction. Mold growth can also cause discoloration and deterioration of the wall. Over time, the materials in the wall like concrete or stone can naturally weather and take on a brown or discolored appearance. In most cases, this is not a structural issue but more of an aesthetic concern.