ink like spots

[saBBfanBoy]

Guys I have a black lumia 920 and I've been using this phone cover since the phone was like a week old and now the is about 2months old and about Week and a half ago I've discovered black ink like spots on the top half on the phone and today I discovered the same spots on the bottom half of the phone. Has anyone had the same problem? What's causing it and HOW CAN I GET RID OF THE SPOTS.

 

[DBDev]

I noticed the same thing when I removed the cover from my 800 and they are still left a bit after using it without cover for over half a year...