You can't selectively hide a custom setting value from users. As such, you should use the advice given in Storing Sensitive Data:
Custom settings enable application developers to create custom sets of data, as well as create and associate custom data for an organization, profile, or specific user. However, setting the visibility of the Custom Setting Definition to “Protected” and including it in a managed package ensures that it’s only accessible programmatically via Apex code that exists within your package. This is useful for secrets that need to generated at install time or initialized by the admin user.
Unlike custom metadata types, custom settings can be updated at runtime in your Apex class, but cannot be updated via the Metadata Api.
Simply by marking your Custom Settings as Protected will prevent even Administrators from viewing or modifying the values directly in Setup. This way, you, the developer, control access to those settings by providing them with a Visualforce page or Lightning component that calls Apex Code in your package's namespace to configure the values. Your code, including classes and triggers, can access the data normally, but it will be protected from outside, including from all other installed managed packages and all users.
I strongly advise that you read the document linked above for additional information and precautions you should take.