My opinion: It isn't worth it to do anything (especially because you only have a common law trademark). Even if you had the federal trademark, it might not even be worth it. A trademark dispute can easily be $100K and you won't get your fees paid for unless the case is particularly egregious.

Of course, someone will tell you talk to a lawyer. Getting legal advice from a member of the legal cartel is a crapshoot. There are so many terrible lawyers out there who won't hesitate to recommend legal action for every little thing even if it has zero chance of succeeding (I am currently on the other end of that). So proceed with caution. Do your own research on common law trademark rights before you talk to a lawyer. That will help you weed out the incompetent ones.

I would keep an eye out if they file a trademark application (check TESS periodically). You'll want to oppose it. That should "only" cost you a few thousand dollars.