Hey there,
I'm not very clear on what you are saying but from what i can gather you want rid of the grey on the bottom of the sphere?
did you try unchecking final gather cast on the plane render stats? I'm quite crap at working out what the problem is if i don't have the scene in front of me.

btw, if you ask me your FG setting are massively high and you don't need to use a "useBackground" shader.