What the coastal climate is doing to that wall
Hermosa Beach doesn't get much rain, but it gets a huge amount of moisture in the air. The marine layer sits over the South Bay for weeks at a stretch, especially in late spring, and that humidity works its way into stucco, block, and old wood framing whether or not a drop of rain ever falls. Add in salt-laden air corroding old pipe fittings and flashing, and you have a slow, quiet process that has nothing to do with a single leak event.
This matters for the blame question. A roof leak points fairly clearly to one unit. A wall that stays damp because of ambient humidity and poor ventilation can build up on both sides at once, independent of each other, which is why two neighbors can both be right and both be wrong about whose problem it is.
More on this from Reading List How To Tell Whose Side A Damp Patch On A Shared Wall Came From.
What you can check yourself first
Start with the obvious mechanical explanations before anything more complicated. Look at your side of the wall for a plumbing line, a water heater, or a bathroom vent that terminates nearby. Check whether the stain follows a straight line down from a window or roof penetration, which suggests a specific entry point rather than general humidity.
Feel the wall at different times of day. If it is cool and damp mainly in the morning and dries out by afternoon, that tracks with condensation from temperature swings, common in these older buildings with little to no wall insulation. If it stays damp regardless of time of day or season, that points toward a persistent source, a slow pipe leak or a gap in flashing, rather than climate alone.
Look outside too. Many of these homes have shared roof lines or gutters that funnel water toward the party wall from one side only. A clogged gutter above your neighbor's portion of roof can send water down inside a wall you both share, and from indoors it will look like it belongs to you.
Where a homeowner's checking stops being enough
This is where air quality testing earns its place, not as a diagnostic for the leak itself, but for figuring out what the moisture has already done and whether it's still active. A tester can sample the air on both sides of the wall and compare spore counts and moisture readings at different depths in the material. That comparison, taken from both units at once, is often the only way to establish that the source is genuinely shared, or that it's isolated to one side.
This kind of testing also matters because mold behind a party wall does not respect property lines. Spores move through wall cavities, shared attic space, and HVAC returns. Two neighbors can smell nothing different from each other while the actual growth sits mostly in one unit's wall cavity. Visual inspection alone, even by someone experienced, can't settle that.
When to call someone in Hermosa specifically
If the stain has been there more than a few weeks, if it's spreading rather than staying the same size, or if either household is dealing with unexplained coughing, headaches, or allergy flare ups that ease when they leave the house, that's the point to bring in an air quality tester along with a leak specialist. Given how many of these South Bay duplexes were built before flashing standards improved, a plumber or roofer may need to open the wall anyway, and having air and moisture data going into that conversation makes the repair, and the conversation with the neighbor, a lot less contentious.
One more local wrinkle: because so many of these properties are individually owned units in older buildings rather than modern HOA condos, there is often no management company to mediate who pays for what. Independent documentation of where the moisture and mold actually originate becomes the thing that keeps the disagreement from turning into a bigger dispute than the wall itself deserves.