Say whether the affected area is painted plaster, another finish or a joint beside a tiled wall. Note roughly how far it extends and whether you have seen peeling, staining or a change after cleaning. Include the surrounding room in a photograph where safe. Do not climb up or scrape a patch to find out what is underneath.
Explain when the signs were first noticed, whether they come back and what work has already been tried. Mention wetness after bathing, a reported leak or an extractor that is not operating as expected. Those observations help the enquiry; they do not by themselves establish why the surface is affected.