If you don’t recognize the video, I hope it’s coming from a plugin or your theme, that’s easy to fix. Please attempt to disable all plugins, and switch to the default Twenty Nineteen theme. If the problem goes away, enable them one by one to identify the source of the problem.
If you can install plugins, install Health Check. On the troubleshooting tab, you can click the button to disable all plugins and change the theme for you, while you’re still logged in, without affecting normal visitors to your site.
If that makes no difference, then it’s probably a much less pleasant situation. Carefully follow this guide. When you’re done, you may want to implement some (if not all) of the recommended security measures and start backing up your site.
