If you refer to the splotch near the center of the phone back, it looks to me like the phone is touching the inside of the cover. I have a clear TPU case for my S7E and it sometimes does that. It happens when I don't precisely center the phone in the case.
When two highly glossy surfaces come in contact they produce this rainbow effect. It's a known issue. Very difficult to use clear cases with glossy backed phones.
Solution? A matte finish protector film for the back over which you can apply the case. Or, smudge some fingerprints on the surface of the phone and inside the case then apply them both. The dirt off your fingers will come in between the two surfaces and prevent them from sticking.
I've had this problem with all my recent Samsung phones as well as the Google Pixel with its glass top.