If you rent
Tell the landlord in writing, with dates and photos, and keep a copy. Under California law the landlord's repair duty generally starts once they have notice. Since 2016, visible mold can make a unit substandard when a code or health officer finds it, which is why a city inspection can matter.
Repair-and-deduct and rent withholding have strict conditions, so get advice before trying either.
Rental mold guide