I was taking a shower with my iPhone 8 because I thought it was waterproof. But when I was done my shower I noticed a mark at the bottom left of my screen
I was wondering how I could get ride of it
Take it into an Apple store for evaluation. The iPhone 8 is water resistant, not waterproof. They will advise the best options open to you.
I fixed my the smudge using rice
"Waterproof" and "Water-Resistant" are 2 completely different things.
Use this as a general rule, even if the phone is "advertised" as being the above two things:
It's not either waterproof or water resistant.
Here's Apple's official statement regarding water resistance:
About splash, water, and dust resistance of iPhone 7 and later - Apple Support
Unfortunately you will most likely need to pay for an out-of-warranty replacement unless you can live with the mark.
Water marks in my iPhone 8 screen
