Mold points to a deeper problem
Serious mold can make a California rental uninhabitable, especially when it grows from leaks or moisture the landlord failed to fix. Mold itself usually points to an underlying problem — a roof leak, a plumbing leak, or poor ventilation — that the landlord is responsible for repairing.
Widespread or recurring mold can affect your health and breach the implied warranty of habitability, the automatic promise that your home will be fit to live in. What usually matters is the source of the moisture and whether the landlord acted after you reported it. A small spot you can wipe away is different from mold that keeps returning because the leak behind it was never repaired.
Keeping dated photos of the mold and its source, along with any medical records if it affected your health, helps show the impact. Do not throw those records away. If mold is affecting your home or your health, a free review can tell you where you stand.
This is general information about California tenant rights, not legal advice. Every situation is different.