Rising Damp or moisture? not sure...

Hi all,
we live on the ground floor of a two storey apartment block.
The apartment is brick and render on the outside.
Recently, in one corner of the floor I have discovered some dark dusty particles appearing along the skirting board.
When I wipe with my finger, it seems to disappear, like dust. Because this wall (that corner of the room) faces the outside world, could it be damp - is this dust consistent with that?
If so, what do we need to do to get rid of it or are we destined to it forever?
(It has rained a bit recently, could that be it? Just moisture? Open some windows?)
Or is it something else? Replies are apprecaited.
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Probably mildew spores.
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