A memory block. There's a brief pause when he remembered using one on Romana. To forget him entirely.

"Memory blocks break down all the time, Clara," he reassured her. "He shouldn't blow up, at least, not directly thanks to the memory block." He didn't, after all, know the reasoning behind the block. For Braxiatel, Romana needed to not remember Pandora because she would have taken her over. The effect of her memories reemerging meant that she had to kill herself, regenerate, to keep Pandora from emerging.