Trapped Souls are souls that've been forced into a vessel that they don't naturally belong to, whether it's by magic or some other means.

[As for the second question, he considers before shaking his head]

I couldn't get close enough to Greed to tell if he had that. [Besides, the thousands of souls would be way more distracting even if he could] I can see dark energies though coming from some people...it usually presents itself as a dark haze, depending on the severity. Is that what you mean?