What is this black stuff coming off my George Forman grill?
Most likely your problem is caused by your dishwasher detergent. Many of them will cause aluminum to oxidize creating the issue you are experiencing.

I would suggest that you hand-wash and remove the oxidation as it looks quite extreme. (You don't want any of it to come off in the dishwasher and be deposited on other dishes.)

While it technically may be 'dishwasher safe', to prevent the oxidation from reoccurring I would recommend going forward that you hand-wash. The other option may be to try different dishwasher detergents to see if you can find one that does not cause this reaction.