What should I do if I believe my former advisor is alleging to journal editors that I have copied his study?
I would talk to the conciliation board on your university and to the editor . Ask your colleague, if he/she will confirms that information in front of the conciliation board. Furthermore, I would do some research (or hire someone) about code of conduct on your university valid this case. Last but not least I would write down every single accusing in a chronological form (kind of factual report).

Maybe you can prove by the timestamps of your files, that he copied ideas from you and not the other way.