I also have the exact same model and the same issue. I'm sure that it's an oxidation layer that is likely caused by the dishwasher detergents. It's however not preventable as its an unwanted side effect of this particular kind of aluminum alloy. I had some pots that exhibited the exact same problem. I got rid of them all because they drove me crazy. I paid some extra bucks for a quality set of pots that have a better aluminum alloy that won't corrode.
Unfortunately, the ceramic coating got damaged at the small dotted line in the middle lower plate due to repeated mechanical contact with the top. It is a design flaw that happened twice for me in the last 2 years. As this oxidized aluminum is not healthy, I have to throw it away. I'm done with this grill and the brand until they prove that they can create a lasting product that is also safe to use.
One last tip: I did a custom modification to the plates. I removed the two handles and put some masking tape around the edges of the plates. I then spray painted the plates with a black coloured extreme high heat resistant paint. The paint is usually available in most hardware stores. Pick the highest heat rated one and put several layers on the backside. This locked the oxidized layer inside and my hand/dishwasher never got soiled after that. ....until the ceramic coating chipped away duh.
You take it or leave it, I'm done with this for good.