The pictures aren't great but it looks a bit like iron oxide, possibly from corroding reinforcement in the concrete. It could be mould due to damp due to condensation but the link to rain suggests that you have a faulty (or absent) damp-proofing membrane.
If I'm right, it's not good news. If it were my building, I'd consult a Chartered Surveyor to confirm/correct my guess and get them to draw up a specification for the repairs, which are going to need a builder who is quite competent.
I hope I'm wrong and it turns out to be condensation. How is the room heated and ventilated?
