From the photo it looks a bit like root mealy bug but I've never heard of it affecting brassicas. Try painting a bit with meths and see it it moves it.

That is a classic image of Root aphids. Much the same as the ones found on Roses, but these live underground. They secrete a waxy substance to protect themselves and that is what you can see as the 'ash.' Not easy to deal with on edible crops, but Ultimate Bug killer as a soil drench will work on non-edibles.

Hmm CHEERS