Hi there,
Hallucinations definitely are scary! When I was manic, I would see shadows as well, but not through mirrors. I saw them mostly at night, going along the walls. I would also see "shadow people" randomly too.
When I was put on seroquel, these hallucinations went away. Definitely talk to your pdoc about this if you keep seeing these, and you know for sure they're not just a trick of the light or something.
Keep us posted on how you're doing, OK?