I think the expression on Ayumi's face on ayu99persen avatar is very adequate to some conclusions in this thread, lol
My theory is, the white thing has no meaning at all, just that they needed some sort of theme to the whole video (the white comes out through pipes, from the top of the screen) and something that looks pretty effective (the dancers splashed with the white, Ayumi 'washing' her hands with it). That's probably why they used it. Kinda like the 'chocolate' in Moments PV.