A concern troll is a false flag pseudonym created by a user whose actual point of view is opposed to the one that the user's sockpuppet claims to hold. The concern troll posts in web forums devoted to its declared point of view and attempts to sway the group's actions or opinions while claiming to share their goals, but with professed "concerns". The goal is to sow fear, uncertainty and doubt within the group.
http://en.wikipedia.org/wiki/Troll_%28Internet%29#Concern_troll