An eye he didn't even realize was closed suddenly opened, and he attempted to regain control, to reassimilate--

He pulls away from Kaworu, perhaps a bit forcefully, but not because he didn't want to hug him.

Instead, his body collapsed once he was clear of Kaworu, leaving nothing but a floating, heart-shaped glow--Mettaton's core, which had phased out of its container. But...then the shape begins to blossom into something larger, more transparent...the form of a silly, stereotypical ghost.