Give the location, surface material and extent, with a wider photograph and accessible detail. Say when growth appeared or returned, and mention dampness, earlier products and known repairs. A recurring patch or suspected hidden concern is more than an ordinary dirt description. You do not need to choose a treatment method before calling, but the enquiry should make clear that mould and moisture are the concern rather than assuming a regular cleaning visit will cover them.
For separate household tasks, see one-off domestic cleaning. Agree those tasks independently so they are not confused with assessment and suitable mould work.