Legal Question in Landlord & Tenant Law in California

It depends on the cause of the mold. Mold that is not inside the walls or fixtures due to leaky pipes or other sources of water that come from needed repairs to the building itself may be an issue for the occupant to take care of through better cleaning and ventilation practices. If the unit itself is impossible to vetilate properly that may be a habitablility issue, which is the landlord's responsibility. If you think the cause of the mold requires landlord repairs to the building, call your local code enforcement agency to inspect it. If they agree they will issue the landlord a citation.