My former post-doc host failed to credit my work in a paper - what to do?
First, please remember that human memory is fallible. People have honestly forgotten work done by their students or their postdocs or even themselves. So I'd be inclined to assume, unless there's evidence to the contrary, that your supervisor has forgotten your work on the broader concept Y that subsumes the &quot;new&quot; concept X, and that he hasn't had time to study the arXiv preprint that you sent him.
Second, what should you do about it? I think that, under the assumption that no malice was intended, you could write your former supervisor an email along the lines of &quot;It seems to me that the concept X in your paper &quot;[title]&quot; is an instance of the general concept Y studied in my paper &quot;[title]&quot; [arXiv link] which I sent you a preprint of a while ago. I hope that my work on Y will be useful to you if you contiue to explore X.&quot; It seems to me that something like this should alert him to pay attention to Y and cite you in further work on X.  (Of course, you can put personal things like &quot;I hope you are well&quot; before and/or after the main message.)
I think you're unlikely to get a citation in the already published paper (unless it has an on-line version that gets updated), but if your supervisor produces more papers about X, you might get some citations in them.