What is this white powder on my grill? I have a ceramic grill that I have only ever cooked organic chicken breast and organic steak on.  I have occasionally added spice but not for months as it makes too much mess.  After cooking, I drizzle warm water mixed with washing up liquid on it from a soft cloth, whilst the grill is still hot (generating a lot of steam), then wipe it in order to clean it efficiently.

After I have cleaned the grill and it starts to cool, I have noticed white powder on it, as shown below.





I can scratch this off gently with my thumbnail and it comes off cleanly, as shown by the highlighted area below:



This leaves a black substance under my nail that has a slight meaty smell:



Can anyone suggest what this is?  My first thought was salt, but I don't add any to the meat (or is there enough in it to cause this?).  Or could it be something to do with the washing up liquid I use to clean it?

(This is my first question here, be nice.  And yes, I know I could write to the manufacturer, but trust impartial individuals on here just as much :))
Not sure anyone would be able to tell you definitively, but, if you add water after each use, my guess would be minerals from your water.