Black Zetsu Madara's will ... White Zetsu Hashiramas will?
As we have seen in the newest chapter, Madara has actualle been fatally injured by Hashirama ...
Several theories as to how he might have survived are already being thought of ...
I for my part believe Madara had help from Hashirama ... To be more precise ... Hashirama really didn't want to kill Madara so his "love" toward his friend came to life from the remnant of his Mokuton Jutsu and white zetsu formed..
From what we know of Zetsu, it is a copy of Hashirama but actually it is a Mokuton creature as through resonating with life energy, it begins to grow shoots.
Zetsu could have been born from the wish to stop his friend without killing him.