It's the WoT score, already present in Coracle. It's not %, it is "equal to how many people you follow that also follow a given person, minus pow(2, log(n)), where n is how many people you follow who have muted this person. This allows you to see at a glance if someone is accepted in your network. This helps reduce spam, impostors, and objectionable content. You can set a minimum web of trust score on your content settings page, which will automatically mute anyone with a lower score than your threshold." (current tooltip) The half circle is more prominent (orange) to show that the user is followed, dimmed for not followed users. I would like to use the coracle sign, (half)circle, more frequently in the interface.