White powder around the diff?

[RC-Monster Mike]

I wouldn't sweat it too much - if it was dust from the slipper pads, it is likely just some break-in dust as you said it was only run a couple times. If you are happy with the slip level and the unit isn't excessively hot, then run it. More slip will equal faster wear(the Teflon bushing is the first and primary wear item), but don't be afraid to use it.
I run mine on the tight side for minimal wear, while protecting the drive train, but in slippery conditions or when the race is on, I tune it as needed to maximize my lap times.