In an argument (usually a political debate), a concern troll is someone who is on one side of the discussion, but pretends to be a supporter of the other side with "concerns". The idea behind this is that your opponents will take your arguments more seriously if they think you're an ally.
A concern troll visits sites of an opposing ideology and offers advice on how they could "improve" things, either in their tactical use of rhetoric, site rules, or with more philosophical consistency. The "improvements" are almost exclusively intended to be less effective.
A typical formulation might involve the troll's invocation of a site's espoused ideals alongside a perceived example of hypocrisy (such as contrasting "we value free speech" with the banning of a "dissenter"), and with a call for some relevant reform by the troll. This reform will frequently be burdensome or silly - the concern troll's message is: "I have some concerns about your methods. If you did these things to make your message less effective, it would be more effective."
Here we can see mouse concern trolling by framing a woman he was abusing as being the real abuser:
Here we can see mouse trying to blame raddle's culture for his own abuse:
Even going as far as to blame people for being oppressed by him ("raddle has made me a nervous wreck") and claiming we are all pushing him away from anarchism and ruining his experience.
Then after a long 'absence' apparently spent stalking the admins, he decides to voice more concerns he has about the viability of the community because of possible alt-accounts:
And is then promptly exposed for using alts to troll.