Homura starts to feel more and more on edge as she watches Mami step to her feet. The numbing effect of happiness, her oblivious state. It's blown away in an instant.

Mami's expressionless face, her quiet and intense voice. ... It's enough to send a chill down her spine, enough make her blood run cold. She thought she would be used to this sort of thing by now. But she isn't.

And then she catches it. Mami's soul gem: a swirl of ugly color through the bright yellow. She no longer looks happy. Instead she looks her normal, stoic self. Her eyes narrow just slightly.

"... Your soul gem. You have a grief seed right there; purify it at once."