Q:

Clay has a scar in the present to differentiate from the past. Is there a name for such a technique?

In the series, '13 reasons why', they make sure that clay gets a bandage on the forehead and stays there so that we can differentiate easily whether it's the flashback or present. 
Is there a name for this technique? I mean, not just the lighting and the color, details like the scar, and maybe stuff that my conscious mind can't recall.

A:

There are no specific techniques for these details. It's all part of the same technique called Flashback.

In movies and television, several camera techniques and special effects have evolved to alert the viewer that the action shown is a flashback or flashforward; for example, the edges of the picture may be deliberately blurred, photography may be jarring or choppy, or unusual coloration or sepia tone, or monochrome when most of the story is in full color, may be used.

As seen above, flashback can use multiple means of telling the viewer that the current scene is a flashback, including Clay's scar.

