Your first steps after finding mold
If you find mold, report it to your landlord in writing right away, take dated photos, and keep copies of everything — then avoid disturbing the mold while you wait for it to be properly addressed.
A written report, like a text or email you save, creates a record of when your landlord was notified, which matters a great deal if the problem drags on. Photograph the mold and, if you can find it, its source — a leak, a stain, or a damp spot. Hold on to medical records if anyone's health was affected, and keep receipts for any belongings you had to throw away.
Just as important is what not to do: do not paint over mold, and do not tear open walls yourself, since both can spread spores and destroy evidence. Let the landlord arrange proper cleanup that fixes the moisture behind it. If your landlord ignores a written mold report, a free review can help you decide what to do next.
This is general information about California tenant rights, not legal advice. Every situation is different.