My ex husband is trying to trademark my photography pen name.?

I have been divorced for over two years now. I have been running my photography under a specific name that I came up with over five years ago. It's my pen name. I created it before we were married, just by about 3 months. I have two photo accounts set up from the time I created it, again before I was married. He has never once even dabbled in the photography world. I get a text this morning, saying he created the name and is taking steps to Trade Mark it. He has never once done ANYTHING with this name at all. All accounts and everything that has EVER been done with this name has been under my real name. He is just doing it as a last stitch effort to screw with me. What can I do? I did not trade mark the name due to it was just fun side project that has recently lead me to be published. No pay, just getting published. What should I do? Any help would be tremendously appreciated.