Well, you have a nice big tank with plenty of space. My best guess is that your fish is just kind of a bully.
Sometimes when 1 fish is getting bigger and hits fishy puberty first, it will get overly territorial because it has mating on the mind and the other fish isn't seen as a potential mate since it is too young. Thus, the older fish will bully the younger fish trying to get it out of the way so better mating candidate can come along.
At this point, I would suggest separating them. That does does look like it might be stressed, and gbr (especially the electric blue) are not a very hardy fish in most cases. You can't just tell fish to get along, and these guys clearly have enough space to avoid each other if they wished. You can wait it out and see what happens, but that might not end well for the stressed fish. Of course, you are the one looking at them everyday to know how bad the situation is.