Since the ball was yet to be thrown, and a number of possibilities existed ;
1. Could be a delayed screen pass to a back following a
path created by the offensive player who had advanced.
2. Could be a delayed hand-off, to a back, running off
left tackle seeking down-field support.
3. Could be a scramble.

considering the offensive player was between the defender and his teammate in possession of a live ball (by definition "a runner", until he might SUBSEQUENTLY become a "passer"), and very easily could have turned up-field to lead interference had one of the alternate possibilities developed and the contact didn't seem overly, or unnecessarily aggressive, I've got nothing.

not even a block in the back?
Wow!

Did the offensive player turn into the contact? By any chance, you wouldn't be an "offensive" asst coach, would you?

Nope. I might have something else, but not a block in the back.

Peace

I coached Linebackers and was a DC for 5 years coached OL too.

so if he is moing away it is IUH according to 9-2-3d

There's a few things that might be called for here. The better view is the end zone shot at the end of the clip.

The hit is made with the ball in the air, so DPI is a possibility. In codes where the possibility of the contacted player's catching the ball is an issue, it's not DPI because it's obvious that receiver could not have caught that ball. In Fed, however, it could be said that the contact prevented the receiver from moving toward the flight of the ball. However, since the receiver had settled & turned around, I don't think the hit prevented that either. So no DPI.

Is it an illegal block for contacting the opponent in the back (with hands in this case)? The opponent presented the back, true, but the defender had plenty of time to react to that. The white receiver did seem to be backing up toward the position of red #32, but that movement was slight, and #32 definitely produced the contact. So I would have an illegal contact there. And that has nothing to do w whether he was eligible to receive a pass or even whether a pass was still possible on the play.

But what I'm really seeing is something more serious: a gratuitous blind shot by #32 when he sees it will be away from the play. He waits for the ball to be released, then hits the opponent; both of them can see the pass, neither of them expect to be part of the play. So the white receiver can be expected to have relaxed & provided an easy target. And maybe #32 even thinks no official will be looking that way, even though the black hat momentarily turns toward him after the hit. But the worst part about it is, he's cheap-shotting a teammate in a lousy intrasquad scrimmage! But then, that's where some of the nastiest stuff goes on, in my experience with children. They're practicing & competing around & with each other all the time, & animosities develop to a greater degree than they'd get a chance to vs. opponents in games. #32 helps him up afterward, and I don't think there was any intention to injure or even intimidate, but I'm guessing he was sending a message. So I've got a personal foul, contact that's unnecessary & may tend to provoke roughness, however Fed calls it, superseding the 10-yarder.