User blog comment:FairyRaven/Fairy Tail Chapter 399: Wings of Despair, Review and Commentary/@comment-7418318-20140908160616/@comment-7418318-20140910075825

Which lends a bit more credibility to the theory that F-Rogue said "anata wa", which would fit the lip movement.

If it was the name of anyone, Natsu would probably try to talk to that person, but if he himself is supposed to kill Frosch, he just might think he'll be able to handle the matter without speaking to anyone about it and it'd also be plenty reason to look surprised.

It might also explain why he attacked Natsu after killing F-Lucy instead of charging after present Lucy.