You should be able to remove the entire spray face of the faucet head. If you'll notice on either side of the aerator there is a flat area where you can grab with a wrench or pliers to twist off the entire face.
Then I would flush the head with water to remove any debris, clean the aerator and replace. If this doesn't do the trick, you will need a replacement head.
If the faucet was purchased later than around 2001, it should be covered under a lifetime warranty.
You can call Grohe directly and request a new head.