the result you attached does not look like a slice, it seems to me that you used the clip filter. In case you do not want to have the blocks of the cuboids, it does not matter whether you use clip or slice.
You can use the calculator, compute the magnitude of the velocity by sqrt(physVelocity.physVelocity) and then use the threshold filter for the magnitude. Choose the minimum of the magnitude to be small, e.g. 0.00001. Then you just get the geometry domain.