Yes. This applies to Membership clients only. When you enroll a client in a Membership, you have the option to link the record to an existing Square customer record. If the client does not already exist in your Square account, a new customer is created when you add their credit card in MassageBook.
Any changes you make to credit cards on file within the membership will be reflected in Square as well. This means that if you add or delete a credit card in MassageBook, the credit card will be added or deleted in Square as well, and vice versa.