Hello,
I checked both extensions on the store.
They seem very similar, and the name is certainly almost identical.
I will have to be honest with you, I don't expect Google to act as a Intelectual Property judge in cases like this, or even, I don't think google can take down the extension if it is not violating any of the Chrome Web Store terms, and it could potentially be sued for that.
To my understating, you will have to legally claim your rights.
It is important now for your to understand what your rights are by law, so I would advise you to seek help of a specialized lawyer.
Big disclaimer now, I am not a lawyer and I can't give you any kind of legally binding advice, I can only tell you what I should do if I were in this situation:
I would first try to contact the owner of the extension, and try to reach an agreement (everything recorded in written form, so that it can be used later in case you go to court).
If that does not work (and based on your description, I am assuming it will not), then I would seek legal advice.
I hope this helps somehow.
I wish you good luck.
Best Regards,
Guilherme.