Greyson wanted to reply to Mariel that she was right, he wasn't a Seer, but he didn't exactly want to be one anyway. He really couldn't understand all the emotions and feelings and drama and confusion that seemed to envelop teenagers, as he himself didn't really relate to those things. While he had plenty to say, he could sense that the Hufflepuff was getting more and more frustrated, so he just stood there awkwardly - looking up and down the corridor - while Mariel continued on with the acceptance of her ability.
When she stood up with an apology and started down the corridor, Greyson found himself unsure of what to do. His natural inclination was to just shrug the situation off and continue on with his night, but he knew that wasn't what he was supposed to do. "Um . . ." he said scratching the back of his head as he watched her walk away burdened.
Greyson turned in the opposite direction and continued on his way, turning over his shoulder once to start to say something before turning back and continuing to walk. It was only at the end of the corridor before the turn he'd need to make toward one of the staircases that he called over his shoulder. "See ya around." Clearly not understanding the need for gentleness or understanding, he treated the interaction like nothing had happened and then disappeared out of sight.