Hi, sadly once the green line appears, it's impossible for any fix to undo the damage. The only solution will be to replace the LCD. Third party mobile repair shops may offer a cheap repair cost, if u are thinking to continue using ur Z Flip 3.
My Flip 4 just about 1 year the "green thing" already came out. Should be still under warranty but they said it's not covered under impact damage. Luckily managed to negotiate and make them waive the charge. Very lousy Samsung service. Took about 2 weeks to settle.
