Welcome to Cruising and Cruise Critic. To be able to answer your questions, we would need to know if you received Refundable or Non-Refundable Onboard Credit.
When NCL gives OBC as a promo item, it is non-refundable! However, if you used a travel agency, you would need to inquire with that person/agency if the credit is Refundable or Non-refundable.
Non-Refundable OBC can only be used to purchase goods and services once onboard the ship, but cannot be used for Daily Service Charges! Non-refundable can be used for food and beverages and most onboard activities.
And to answer your question about does it automatically come off, Yes! Generally speaking, the Non-Refundable OBC will be applied to most things onboard, first, then any Refundable OBC, and then your form of payment onboard. Hopefully, this information helps answer your questions.