Posted: 17/10/2016 at 22:41
Yes fungus is a possibility, but it's easy to tell the difference. Fungus mycelium ( 'root') is stringy in appearance and when rubbed between the fingers will go mushy. It is usually brilliant white in appearance. If it remains dry and dust-like and grey or yellowish it is minerals. Also it is unusual for fungus to cover very large areas. It tends to form in discrete patches. You will also probably have noticed the fungi above ground around about this time of year.
Yet another possibility is cement from old buildings!!