Who is expected to pay for finding the mold
A professional mold inspection can help document a problem and locate its source. In a rental, the landlord is generally expected to investigate mold they are responsible for — including any inspection needed to find where the moisture is coming from.
Because mold usually stems from a leak or dampness the landlord must repair, the cost of finding and fixing it typically falls on the landlord, not the tenant. That said, some tenants choose to pay for their own independent inspection to create evidence they control, which can be useful if a dispute develops later. Whether that is worth the money depends on your situation.
If you do get an inspection, keep the report along with your photos and your written reports to the landlord — together they build a strong record. If a landlord refuses to investigate obvious mold or pushes the cost onto you unfairly, a free review can help you understand what is reasonable and what your options are.
This is general information about California tenant rights, not legal advice. Every situation is different.