Give the room, surface material, approximate extent and timing. An overall photograph places the marks in context; a detail shows the visible condition. Explain recurrence, wetness and any earlier products or repairs. Do not assign a species, health diagnosis or moisture cause from a dark colour. If you are unsure what the surface change is, identify that uncertainty and ask about appropriate inspection rather than choosing a remedy before the enquiry.
A one-off clean for normal household dirt has a different scope. Mould and moisture concerns need their own assessment request instead of being treated as ordinary grime.