iPhone stains marks. How to remove it?

I have an iPhone 7 which had a cheap transparent silicon cover from the moment I bought it. when I wanted to change it now, I was surprised to see the back of my matt black phone 7 to be covered with small rounded stains that does not look serious but it can't be removed. I tried to use wet tissue as it doesn't look serious, but not. I used a bit of screen cleaner in the tissue, still no use.

since everyone uses silicon transparent cover, I figured someone had already faced this issue and solved it, thus starting a new thread asking.

Thank you so much guys. sorry for the bad pictures. I have no other phone to take photos from, hence used my MacBook.