Strange circular USB icon in Finder

These icons are a bug caused by the app What's Your Sign?. Uninstall the app, reboot, and your Finder sidebar volume icons should return to normal. There's an open issue for this bug at the What's Your Sign? github so it doesn't appear as if the issue is fixed even in the most recent release.

I have those now, too. I assumed the sidebar icon had just been changed to those circles in a recent update.

As far as I know, the default sidebar icons are still located here:

/System/Library/CoreServices/CoreTypes.bundle/Contents/Resources/

You should be able to swap any or all of them out for any icons you want. Just give your new icon the same name as the one you are replacing and make sure it is a .icns file. You will probably need to disable SIP in order to do this.

EDIT: On closer look, I don't see any dots amongst the default sidebar icons in CoreTypes. I wonder if it may have something to do with disk permissions....