Andrew,
I agree with Roger to a point, but here is another thought. 'White'
is odd. Yeast debris in a red is usually pink, tan in a white wine.
If you added something to the wine maybe that's what it is.

With all that CO2 in there it's hard to imagine any bacterial
infection but I can't think of anything that 'should' be white in a
wine.

I would wipe it out, or if it's a film on top, just top up to
overflowing and let it get rid of itself. Then get some wine back out
if necessary and shake the carboy to fill the headspace back up with
CO2.

Joe