A non-closed circle icon in macOS menu bar that I cannot remove

Solution 1:

Assuming you just want to remove it, you could check your Login items:

Go to Apple > System Preferences > Users & Groups
Make sure your User Account is selected at top left
Click on the Login Items tab
Look for any items listed in the list of login items
Now remove any items you don't want to start automatically by clicking on its name (so it's highlighted) and then clicking on the minus - button
Once it's removed, exit System Preferences
Restart your Mac to test to see if the icon is still there