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.

It's definitely oxidation. I bought a new dishwasher recently and that same thing is now happening to all the outside bottoms of my nonstick pots & pans. At the same time I switched from powder dishwasher detergent to those muti-colored liquid pouches, so I'm not sure if it's the new dishwasher which gets hotter than the old one or if it's the new detergent or a combination of both. Super annoying. The reason most people own a dishwasher is so they don't have to wash things by hand... duh.

I believe it is the heating elements in the grill that are oxidizing... grill gets way hotter than dishwasher. Also the heating elements are covered in the same soot. I don't think there is a solution but wonder how safe it is to continue cooking with the unit.

I agree that it is the heating element on the grill producing the black soot. Neither dish washing or handwashing makes it better or worse.