The WordPress SEO plugin should not have a Backdoor Script in it. If my plugin is detecting a Backdoor in any of those files then it must have been injected by hackers. You can click on the file names in red, that come up in my plugin's scan results, to see the contents of the infected file. Then click one of the linked numbers to highlight the malicious code in that file. This is the code that will be removed if you use the automatic fix button.
Please let me know if you need any more help.
Aloha,
Eli