It's lucky that Sam is actually shaking Steve's hand before what Quorra said clicks with the name and he nearly drops it.

"You're Captain America." Luckily for Steve it's not said in a tone of fanboying but the flat voice of someone who just can't believe the world. It takes a lot to take Sam by surprise now, and having Captain America show up on his doorstep well. He can deal with that.

Except that he can't. Not really, which is why he turns to Quorra and says "You got a phone-call from Captain America?"