This is a minor issue, and one which I resolved long ago, but one which has bugged me for ages. How do you apply a possessive to a generic descriptions. Specifically, do you say "someone else's" or "everyone's". Every time I do, the damn spell checkers mark it as an error, but I never knew why.
According to Pain in the English, it's perfectly acceptable (apparently the spell checker assumes you're a moron, assuming you intended "someone elses" (plural), and just added the punctuation as a good luck charm.
So, for all you confused by your grammar checker, wonder no more. "Else's" is perfectly legitimate.
You'd think, for as long as grammar checkers have been around, someone would have figured out that this is not and error and corrected it, but there's never been any widespread attempts to improve the accuracy of grammar checkers over time. Instead, each company simply dumps a beta-version grammar checker onto their product, figures 'that's good enough', and promptly forgets it.