What makes this scene so nice is the fact that Mack doesn’t even try to guess the right word. He’s not doing what Imaginary Simmons did in similar situations. He’s not telling Fitz to be patient, he’s not telling him to take his time. He’s not telling him he will remember the right expression in time.

No.

Mack is basically saying that the exact phrase doesn’t matter to him. That he doesn’t care about all these words Fitz can’t remember. That it’s okay to not understand somethnig.

This scene shows that Fitz is still capable of doing awesome things, even if he can’t communicate like he used to. And that Mac, as a person who hasn’t met Fitz before, doesn’t have a problem with it. 

It’s nice to see that someone finally started treating Fitz like a thinking person, in contrast to the rest of the team: worring about his mental state and being overprotective in every interaction (I mean, ok, it’s sweet of them, but Fitz is well aware that he’s not the same, he doesn’t need a constant reminder about it)