Cleaning task or landlord repair?
It depends on the cause. Small amounts of surface mold from everyday condensation are often the tenant's to wipe away, but mold that keeps returning because of a leak, a broken fan, or poor ventilation is usually the landlord's responsibility.
Bathrooms and windows are common spots for mold because they get damp. If mold appears only because a vent fan is never used or a window is never opened, cleaning it is a normal part of living in the home. But when the exhaust fan is broken, a window will not seal, or a hidden leak keeps the area wet, the underlying problem belongs to the landlord — and so does the mold that will not stay gone without that repair.
The key question is whether ordinary cleaning solves it or the mold keeps coming back. Recurring mold in the same spot is a sign of a moisture problem that needs a real fix. If a landlord blames you for mold that clearly traces back to a broken fixture, it may be worth getting guidance.
This is general information about California tenant rights, not legal advice. Every situation is different.