Alerting your landlord of the issue of mould within your/their property will most likely take place through third party correspondence with your property manager.
It’s best to send an email with photo evidence of the mould and where it is occurring. Most property managers will have a guide to advise how you can begin to deal with the mould. If the issue of mould is coming from a structural issue within the property (ie, a leak) your landlord will need to resolve this.