I have just put in a new staircase and the underside was filled in with wire mesh and then plastered - approx 10 days ago. The plaster has now dried out except for four or five spots which still appear damp and dark in colour. A couple of the spots have sprouted a white furry bloom. There are no pipes nearby and it is some way from an external wall. Does anyone have any thoughts about what it might be? Would apprciate any ideas or suggestions. Thanks.