It depends:

• Small spots in bathroom, around windows, corners that appear due to daily living (steam, condensation) and lack of ventilation

Tenants are expected to clean these quickly using normal household cleaning products (and to improve ventilation habits).

• Large, persistent mould that keeps returning even when you ventilate and clean properly

You should report this to the landlord with photos. If it is caused by a structural problem, the landlord is usually responsible for repairs.

If you ignore small mould spots and let them grow for months, this is usually considered negligence, and cleaning/repair costs can be deducted from the deposit.