It looks like damp. How far up the wall is it? Does the wall have a damp-proof course? It looks as though the damp is rising and bringing salts with it which remain when the water evaporates (rather like the way a stalagtite forms) and cause the plaster to come off. I imagine that you could scrape it off with a scraper and that you'd be left with bricks with a slightly crumbly surface. If my reasoning is correct you'd need to get a DPC in somehow. This could be injected by either yourself or a professional firm.
Rob Graham