What can I do when some people stole my master thesis work and published it as their own work?
I have this one paper that has already gotten copied and submitted to some spam journals 5+ times. It literally happens multiple times per year (it has a catchy title that seems to appeal to a certain class of plagiarizers). I also usually learn about this through a Google Scholar alert.

The first two times it happened, I tried exactly the steps recommended by other answers. I contacted the authors (no reply ever), raised a stink with the journals (no reply ever), and contacted IEEE (the copyright holder of my original paper). IEEE indeed did react that they will look into it after quite some time, but nothing ever came out of it.

My lesson learned was that these things are basically impossible to fight. The authors or journals could not care less about ethical integrity, and for IEEE the measurable damage of one of their papers being "reprinted" in a different title and with slightly different words in an obscure scam journal that nobody ever reads is not large enough so that they would make an effort hunting down who is legally behind these shopfront journals.

The good news is that it is also completely irrelevant to you. I can assure you that your scientific contribution will not in any way be lessened by this paper existing, nobody will read the other paper (because nobody reads these "journals", period). For future promotions etc., the existence of this paper will not be in any way an issue for you. Science as a whole suffers (to a degree that we could argue about), but the fact that it happened to be your paper that they copied (as opposed to a paper by me, or somebody else) makes no difference at all.