Would screen replacement fix green lines/flickering on screen?

Green lines and flickering is usually indicative of physical damage to the LCD or OLED panel underneath the glass, and would most often be resolved by a replacement screen, however with the water damage there may be a little more to it. If the device actually functions completely fine now with no errors or anything you might have gotten lucky. I’d recommend taking out pretty much every component to scrub down with some alcohol based cleaning solution that’s safe for electronics, and then put it all back in with the original screen(also clean connectors of screen flex cables) and see if it’ll work. If the issue is still persisting then replace the screen and everything should be fine!

If possible it may even be worth going to a reputable repair shop to see if they can diagnose whether or not it’s worth getting the screen replaced.