My material appears as a ghost in the menu… But is full working in my Scene.
So in a way it is not a huge problem, only quite annoying.
This happens sometimes, the thumbnail seems to be set based on what is in the actual material. Just open it, compile and save it again, and see if it was changed.
I compiled it several times already 
But after restarting Unreal several times the material thumbnail just appeared. It’s good to know there’s not something wrong in my material. Thanks!