@Eml0140
Poison is my first thought. Also the most likely. If so, he doesn't know the poison is something Nie Li can deal with, so he can use it and let it be thought of as a disease.
My second one is that maybe he thinks he can deny Nie Li access to spirit stones and other leveling tools. Which seems unlikely, but who knows.
Third idea is maybe he wants to harass him, denying him the time to meditate and absorb energy. I also think this is unlikely because he probably don't have this level of authority.