I would normally expect "the grocer's apostrophe" to refer to a single grocer and "the grocers' apostrophe" to refer to a plural group of grocers, which I assume is what you intended.
The term "(green)grocer's apostrophe" refers to the misuse of the apostrophe in plurals, which seemingly occurs disproportionately on signs in those shops. It's ironic that it contains a tricky-to-place apostrophe. Should the meaning be "the apostrophe of the greengrocer" or should it be "the apostrophe that greengrocers misuse"? Either works fine. For the same reason, I always have to check whether it's "mother's day" or "mothers' day" because... it's both!
