Hah - its indeed the LCC diffuser tool.
Take an images with that in front of the lens, before every session, at the working aperture/ camera/lens combo in use, to:
(a) remove any color casts*
(b) delete any sensor dust
Open up ~2stops (shutter speed, not aperture) to get the right exposure with it.
Its very useful and good workflow practice to do it every session
If you forget, its ok, just use a prior LCC capture, but the sensor dust will likely not be accurate.
apply the LCC to every image in that session, for improved results. It does make a difference!
(*not just wide angle lenses - even standard lenses have some color cast to them as you move away from center)
