How California law treats mold in rentals
There is no California law that bans mold by name or sets a single legal mold limit. Instead, serious mold is treated as a habitability problem — so a landlord who lets significant mold grow and go unfixed may still be violating the law.
California judges mold under the broader rule that rentals must be safe and fit to live in. When mold is widespread, tied to an unrepaired leak, and affecting health, it can breach the implied warranty of habitability even without a mold-specific statute. Local housing and health codes may also treat serious mold as a code violation, and a city inspector can sometimes cite a landlord for it.
So the honest answer is that it is not the word "mold" that makes something illegal — it is a landlord failing to keep the home livable. If mold in your rental is serious and your landlord will not address it, that may cross the legal line, and a free review can help you understand whether it does.
This is general information about California tenant rights, not legal advice. Every situation is different.