<p>Like some other users, Moderators do this occasionally (and manually!), when we notice that some users have a very low acceptance rate and almost no voting activity. Most of the times, our message are written on the fly (as far as I am concerned, at least), although we could resort on our <a href="https://stats.meta.stackexchange.com/q/771/930">Library of helpful responses</a>. This is done in accordance with the <a href="https://stats.stackexchange.com/faq#howtoask">FAQ</a>:</p>

<blockquote>
  <p>This lets other people know that you have received a good answer to your question. Doing this is helpful because it shows other people that you’re getting value from the community. (And if you don’t do this, people will often politely ask you to go back and accept answers for more of your questions!)</p>
</blockquote>

<p>We do have 'post notice' to signal lack of reference or insufficient explanation, but nothing regarding vote or green mark.
The idea of flagging automatically question with such incentive remark is nice, but I think it is already implemented unless I am wrong (I haven't asked a question for a while): "Consider accepting an answer" should appear in the list of questions on the user profile, although it <a href="https://meta.stackexchange.com/q/14594/150510">disappear after one month</a>. Moreover, <a href="https://meta.stackexchange.com/q/87767/150510">accepting an answer is not an obligation</a>.</p>

<p>Other Meta SO threads of interest:</p>

<ul>
<li><a href="https://meta.stackexchange.com/q/10672/150510">Encourage users to select 'Accepted Answer' for old Questions</a></li>
<li><a href="https://meta.stackexchange.com/q/8692/150510">Force Accepted Answers on Questions by Inactive Users</a></li>
<li><a href="https://meta.stackexchange.com/q/12543/150510">Is it bad form to prompt new questioners to formally accept your answer?</a></li>
<li><a href="https://meta.stackexchange.com/q/14994/150510">Do you feel dirty if you nudge new users to accept your answer when they indicate you've answered their question?</a></li>
<li><a href="https://meta.stackexchange.com/q/12583/150510">Do I always have to accept an answer?</a></li>
</ul>
