What are my rights? Or what is the right thing to do

Not sure what you are asking about or why. You do not indicate that you ingested the worm, that there was any injury sustained if you did, or that the worm was not a part of the food product. By example, if you order fish there are commonly various worms in fish flesh, and while most cook away or are cut out, wild caught fish, like grouper and snapper still have them, and as such would not be as problematic as opposed to finding a worm in food where one would not expect to find one such as iced cream. You indicate that they discounted the check, leaving the issue to be "what is it you are expecting or wanting from them at this point?" Are there medical bills you incurred, mental health issues hat have been caused or effected? Was the company response inadequate? If you have specific issues, such as these, you will need to address this by hiring a lawyer to review the issues and provide you specific guidance on any rights or entitlements you may have. That said, if this was some indication of overall sanitation issues, such as the worm actually being something more insidious like a maggot, you might want to consider contating the local health department and making a claim.