When the bank account was originally established by your Mother, both of your names were put on the account, she as the co-owner and you as the minor child. When a financial institution refers to a co-owned bank account as a Surrogate account, it usually refers to a legal situation involving courts and/or an inheritance.

Each state would have rules for how you can access this co-owned account and the bank should be able to assist you by describing the documents and forms you require to withdraw money from the Surrogate account. Do not deal with the teller at the counter, but ask for help from one of the bankers who will have more understanding of the process you must take to access the account funds.