I logged in my Facebook account through the incognito tab but accidentally closed it before logging out. The session still remains logged in... Is there another option to log out from this browser without changing my Facebook password?
If you closed a tab but still have the incognito-mode window itself opened, you can visit Facebook in another tab of the same window and log out from there.
If you closed all incognito windows, all session information in the browser has been destroyed. (That's kind of the whole point of incognito mode.) It doesn't matter whether you clicked "Log out" or not – it is practically impossible for anyone else to use that session again.
If still in doubt, you can visit Facebook's Settings → Security and remotely destroy a session from that page.