Ikea Ringskär single-handle mixer tap scaled / mixing nozzle missing

I have the Ringskär single-lever mixer tap in the kitchen. The aerator or mixing nozzle is now limescaled. However, I cannot unscrew the aerator as I would with other faucets. Can anyone help me? Is there perhaps a special tool?

Thank you very much for the good tip! I was at Ikea and ordered the key there. It was sent to me and it did not cost anything.
Regards