A concern troll is someone who disingenuously visits sites of an opposing ideology to disrupt conversation by offering unwanted advice on how to solve problems which do not really exist. Topics of "concern" usually involve tactical use of rhetoric, site rules, or with more philosophical consistency. The concern troll's posts are almost exclusively intended to derail the normal functions of their targeted website.
Alternatively, a concern troll is someone who enters a discussion with a pre-formed opinion contrary to the majority opinion, but pretends to conform in order to subtly sow dissent and doubt without being called on it. Such attempts often begin with the troll raising "concerns" about the topic of the discussion, hence the name.
While this is a common internet phenomenon, the term is open to misuse. More paranoid circles may accuse someone of being a concern troll for the simple crime of not blindly agreeing with all of the group's dogma.
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."
One common tactic of concern trolls is the "a plague on both your houses" approach, where the concern troll tries to convince people that both sides of the ideological divide are just as bad as each other, and so no one can think themselves "correct" but must engage in endless hedging and caveats. This preys on a willingness to debate critics and allow dissent; everyone wastes time discussing the matter and bending over backwards, so as not to appear intolerant of disagreement, all to the great amusement of the troll.
If the troll provokes a negative response, it can be used to their advantage; the troll can later claim that their victims were unreasonable and closed-minded, doing damage to their reputation. The victims might be so on the defensive against further concern trolling that they end up disregarding and mistreating outsiders with genuine concerns. This makes it similar to sealioning.
Sometimes outsiders will come by and make concern troll-like statements sincerely. Some are sincere but stupid. Some may be insightful, and justified iconoclasts, merely making valid observations unwelcome to the dominant ideology or culture of the forum (the Cassandra syndrome[note 1]). Often, of course, the person is misguided, wrong, and not intending to troll. However, an unsolicited tone argument is, in practical terms, not really any different.