The handseals that Zetsu used confirmed who, of them, was the source of the chakra signature Itachi had followed. He didn't recognize the jutsu, nor its effects. However, he was sure that he didn't want to be on the bad side of the other shinobi, no matter how strange he might be in appearance. At least, some of those strange creatures were on his bad side.

And speaking of which, Itachi was sure that there was something else there, something that was also watching this fight. He wasn't certain of what it was, but he wasn't going to stir it to action, either. After all, it wasn't wise to lay out his entire hand before he knew what the game was.

So, Itachi contented himself to continue watching, intent on going to speak to the ninja and his companion after they had concluded their fight.