What does a half green circle in facebook messenger mean?

Have you tried clicking on it? Maybe it’s a message about the data breach. I haven’t seen that on mine.

Try reading this article, and as you do, go through your Facebook page, explore the links and the drop down menus.

https://www.makeuseof.com/tag/guide-facebook-symbols/

Let us know if you find it.

A half blue circle used to mean they were idle so maybe they changed it from being blue to green?

I just looked at my Messenger and saw that some people had half green circles next to them with a time, i.e. 2 min, in it. So that is showing how recently they were active. The most recently active are in a top group with those currently active and there is a second section below with those who were active a longer time ago such as 20 min.