You probably switched to NS Flex recently. With NS Flex, you don't need any credit on the card to travel, since the travel costs will be debited from your account automatically at the end of each month. Any credit that was on the card when you switched to NS Flex will be paid out to you after the first month of using NS Flex. This is shown in Mijn NS as a negative amount, with the rather unclear description of Customer service.

So to sum up, the amount is the credit that was on the card and is not debited, but credited.

On the card was credit, that debited from cart automatically. Then I switched to NS Flex and on that moment card have a positive balance. Then this amount appears with a “minus”. I still don’t understand why and from this amount appeared. 
 

That's indeed the balance on the card. Because you won't need it anymore with NS Flex we did deduct that amount. You will receive a invoice with this balance as refund.

You loaded NS Flex on your card so you don’t need any balance anymore.

This is why it was removed from the card and will be paid back to you (as a negative amount/credit on your monthly invoice).

Thank you very much, everyone!