If the white color also looking a bit pale or yellowish and your Blue Light Filter is off then this could be an OLED Burn in problem. I am facing a similar issue in my S9+. Its a problem with AMOLED technology.
You can go to PLAY STORE and try OLED Burn In App. Some people have been able to fix this problem but mostly it's a permanent problem that can't be fixed via software. I could not fix mine but you can try your luck.
