So it has
Should I replace u-turn with grass knot? Or if you think I should replace another move let me know also, thanks!
Greninja is mainly a speedy Pokemon, but since it is a water type with dark as it's secondary type, then you should go for special attack moves and have a move that raises it's special defense
Grass Knot is only good for heavier Pokemon. I would suggest getting a stat raising move. I'm not much of an expert because my Pokemon are mostly attackers but do what you are comfortable with.
I am training a greninja and has the above moveset.
Ice beam (coverage and checks grass types)
Knock off ( cus its always necessary, stab)
Gunk shot (screw you fairies) or you might as well have U- turn for switching purposes. Hope this helped as it did to me.