But what grammatical role does "hustle" carry here then?I don't think he's addressing them by the term/name "background". I don't think there's a comma.
He's going down a checklist in his mind, like Dojibear says. Hustle background is his personal shorthand for the name of a step in the checklist. He's basically saying "Hey everyone, we've reached the hustle background step." and then they do what they know they are supposed to do when that step is reached, which involves getting things in the background active and busy, so the scene looks realistic. I think some directors would just simply say "Background!" He's giving them a pre-planned command, so in that sense he's addressing them, but only indirectly, not as an anthropomorphic entity called "background". He could just as easily say "Hustle the background." The people responsible for the background know who they are. Some will be in the scene, some will be operating equipment that adds something to the scene.
Before the scene:
Coordinator: "When the director calls 'Hustle background' your job is to walk across the room and out that door without looking at the camera. You got it?"
Extra: "Yes, I understand."
The director calls out "Hustle background" and the actor does his pre-arranged job, as do all the others in the scene.
I think no matter how "hustle background" is used in a broader context, it itself, these two words, must have a grammatical connection.we've reached the hustle background step.