A term of abuse. Mostly used in forums where there is a fairly well-defined orthodoxy, usually political (it could be conservatism, feminism or nearly anything else) that all members are assumed to agree with by default, "concern troll" can refer to nearly anyone who expresses disagreement or skepticism about some aspect of that orthodoxy, while agreeing with other parts of it.
Supposedly, the idea is that the "concern troll" is actually an adherent of some other, opposing orthodoxy, disingenuously pretending to be sympathetic to the goals of the forum in order to disrupt it or sow dissent. Perhaps this actually happens (this longtime forum user is skeptical, having NEVER seen a clear example), but usually, the accusation comes from someone who can't imagine honest disagreement with his or her favored ideology, and thus says more about the accuser than the accused. Basically it's a bludgeon used by people with very black-and-white views on some topic, against anyone more nuanced than themselves.
Accusing someone of being a concern troll is generally a bad idea. Even if you're right about the "troll"'s motives, which you probably aren't, that doesn't make the "troll"'s arguments wrong; in other words, calling someone a concern troll is a basic ad hominem fallacy. The term is used to shut down, rather than to advance, discussion.
I tried to tell them their statistics were discredited decades ago, but they just shouted me down and called me a concern troll.