White Fluff Substance?

[MrBosc]

Hi all,

Recently my Savannah Monitor past away =( and I now have a new Savannah which I have recently got. However after coming back to my empty vivarium I noticed that this white fluffy substance had grown in the top soil/play sand substrate. It had grown in the left end of the vivarium where the basking light was, on the middle and right end (cold spot) of the vivarium none of this was present. What is it? and should I be concerned to put my new Savannah in there just until the morning until I purchase some new top soil and play sand.