She's the Liara of the game (people who have played the Mass Effect Trilogy understand what I mean). Although, technically I think it's the mysterious artifact that is, but that has nothing to do with her "background".
Saying that, I would suggest people replay the EA as a Cleric of Shar if they want to have the entire picture that is Shadowheart. You have a few unique dialogues with her about her memories/mission. It gives a different picture.
The glow happens where signs of Dark Justiciars vs Selûnites conflict happened:
- a destroyed statue of Selûne,
- a journal mentioning children kidnapped by Sharrans,
- the corpse of a Dark Justiciar,
- after visiting the destroyed Temple.
She does not have that reaction at the untouched Selûne shrine in the Owlbear cave.
Seems to me that whatever that is react to passage of the Dark Justiciars and not Selûne. That would fit with her personal quest which start once you learn about Ketheric, the leader of those Justiciars.