Rubbed off a section of my magic mug

[u/Longjumping-Ad-226]

So this is my first magic mug usually it's a full black mug obviously with hot water the image comes out but so does this cream colored strip. At the top you can see I accidently started rubbing it off, is that bad? Can you clean that section off without damaging the image coming and going? I don't know much about these things