degree sign on screen of my macbook air

If it's not a Safari extension (and that's a good possibility here), is this possibly a window that some Safari web site pop-up or some HTML document has opened? Pop-unders can open windows, for instance. If it's not a window that can be closed with the red button in the upper left, then try close Safari, and re-open it with the command and options keys held down when launching, or(since OS X 10.8 or so) select the setting to disable re-opening existing tabs that's buried in the  > System Preference > General > Close Windows When Quitting setting and close Safari and re-open it and see if the window is still around.

If this is potentially a temperature display up in the Mac menu bar somewhere and not an angle, then there are add-on tools that can be configured and can generate that, and add-on tools can usually be removed. There are posted write-ups on some of the add-on tools that have been around. One option to try if this is in the menu bar is to click on it and see what gets displayed, if anything. The format and contents of any resulting additional display can also sometimes indicate which tool is installed. Removing these add-on tools is obviously specific to which tool was installed.

If this is a floating display of a temperature or maybe of an angle, does the display of that graphic persevere across the shutdown of all currently-running apps, or across a macOS reboot, or into Safe Mode?

Go to Safari Preferences/Extensions and turn all extensions off. Test. If okay, turn the extensions on one by one until you figure out what extension is causing the problem.