Must I know the cause before calling?
Describe observations and known incidents instead of choosing a diagnosis. Appropriate inspection and moisture investigation can consider the actual concern.
What if I do not know the surface material?
Say it is uncertain and provide safely accessible context photographs. Do not scrape or open material merely to identify it for the enquiry.
Should I choose treatment from an online picture?
The actual material, condition, extent and moisture questions need considering. Ask about suitable work rather than selecting a remedy from appearance alone.
Does noticing it today prove it started today?
Give the date you first saw it and identify uncertainty about earlier condition. A first observation is not a complete history of the source.
What should a rented-property occupant do too?
Report the concern to the landlord or managing agent, keeping dates and photographs. A cleaning enquiry should not replace reporting moisture or repair issues.
Does a small patch need a normal clean instead?
Describe mould directly so appropriate assessment and suitable work can be discussed. Ordinary cleaning has a different scope, regardless of the visible size.