It's possible that Grobe thought Harris should have been a bigger participant in practice in the week leading up to Duke. Maybe this is a message to Harris to practice hard today and tomorrow. That would be my guess.
I love Harris, but he's a bit of a primadona. If he doesn't wave off the substitution after the big run against FSU, he doesn't tweak his hammy. Then, he acts like he's been shot twice on the sideline. Then, he tweets after the game he's fine, only he's not. The whole thing is just a little too much me and not enough we, for Grobe's taste (my guess)