I have not run into issues like this, but I would at least make an attempt to meet them in the middle. NDA's and the such are not something I would be confiden

I have not run into issues like this, but I would at least make an attempt to meet them in the middle. NDA's and the such are not something I would be confident in, particularly given that this customer can throw a lot more money and effort into legal. Regarding meeting them in the middle, I would put together a presentation that describes how your algorithm works at a high level. I'd do your best to split the balance between being transparent and focused on the customer, and not divulging what you consider to be differentiating parts of your implementation. If you get push back for the above, I think you're dealing with either brain rape, or some pretty unprofessional contacts in your customer's organization. If it's the latter, you should do your best to navigate around those contacts.