Whether or not Noctis was reacting to him or something Ardyn was doing, Clarus couldn't tell. So he continued to try, lifting both hands and shoving Noctis back with quite a bit of force. If his voice wouldn't work, maybe physical force would.

"This has gone on long enough," he growled, pressing his attack by shoving again. "We need to return to the ship now. We cannot dally here any longer, Noctis!" Clarus had no idea what might happen if they remained in the Dream Width too long, if they would be lost forever, and he had no intention of sticking around to find out. He may have taken a risk when he'd encountered Prompto, but he would do no such thing with Noctis.