Oh wow, really? Well, I hope Slash is for the better. I guess the other move (Heatwave, according to Kyzon?) was a bit slow by comparison.
Speaking of move names, I see a lot of people have trouble identifying Greninja's moves that use water swords. I think it's simply "Cut." I know we don't typically picture Cut looking this way, since it's a generic HM move that's appeared for a variety of Pokémon in every Pokémon game to date... but when you think about it, how would Cut appear when used by Greninja? His Pokedex entry says he makes throwing stars out of "sharpest water," so it makes sense he'd craft a sword the same way when using Cut.
As for his Final Smash, I bet it's Night Slash, one of his high-level moves. In classic ninja fashion, they convey his speed and power as a flurry of hits -- in this case, silhouetted against the moon at night, staying true to his Dark-type.