In some cases, a homeowner's policy will cover mold damage to a home in California, however, it largely depends on what type of policy you have. In areas that are wetter, it is more than likely that your homeowner's policy does cover mold damage because it's a likely inevitability that it was going to happen. There are some cases however, that would not be covered regardless of whether you're homeowner's insurance covers mold damage or not.
The biggest caveat to mold damage is that it must be reported to the insurance immediately once you've noticed it. If it has sat, or it is due to an issue that you knew of, such as a leaking pipe, and you did nothing to fix it while fixing the leaking pipe, then your insurance will not cover the damage repairs, regardless of whether or not your policy covers it. If you find that your roof has been leaking for a while, and you're just now starting to visibly notice mold growth inside the house, your insurance is unlikely to cover the damage because it wasn't reported immediately.
In some cases, the insurance company won't pay out on the policy unless it has been inspected by a professional and an estimate has been given within the appropriate time frame. In most cases, a professional is classified as someone that works in water damage and mold mitigation, such as a property restorator, water technician, or disaster preparedness company. Simply calling the insurance company and saying that you have mold and would like it fixed is not enough. Because mold can be quite hazardous to your held, it should always be treated and cared for by a licensed professional that is trained to deal with it properly to ensure that it is completely removed and treated.