Q: Marks on the back of my Iphone 6s

So I have an iphone 6s 64gb in gold that I purchased around last year in november. It wasn't till last week I started to notice these weird marks on the back of my phone, around antenna band lines and the speaker openings and they do not go away. No matter how I clean it or how much alcohol I use. They stay there, rather they seem to be growing..

My phone is still in warranty for two months. What should I do? Please help!