Overwatering? I had the same problem with a few pots, turned out they were too wet....but you need to be more specific! Is this soil in pots or ground??

The furry growth is a fungus of some kind, growing because the soil is too wet, as Busybee26 says, from overwatering or poor drainage. Once the weather heats up and dries up, in time for the barbecue summer we always enjoy in the balmy UK, then the problem should disappear.