Question
what it is the difference between S.leucophylla "Green" and the S. Schnell's Ghost?

Answer
Schnell's Ghost is a cultivar of the a true green and white Sarracenia leucophylla.  This plant lacks all red pigments.  Normally the flower is red, but on a Schnell's Ghost, the flower is yellow.  When a plant lacks red pigments, it's referred to as heterophylla.  Schnell's Ghost is such a plant.

Sarracenia leucophylla "Green" is similar to Schnell's Ghost.  It produces green and white pitchers.  However, it's not a true heterophylla because it contains some red pigments.  The pigments show up only in the very young growth, in areas of leaf damage and in the flower.  Otherwise, mature pitchers are green and white.  Without the flower, a green leucophylla can easily be mistaken for Schnell's Ghost.  But if you look closely at minor leaf damage, such as from insects, you'll see a margin of red around the damaged area.  You won't see this on Schnell's Ghost.

Good growing!
Jacob Farin