What you DON'T do is ignore this hoping it fixes itself, because green lines on iPhones almost always get worse. Don't try to "fix" it yourself with YouTube tutorials either, because that's gonna seriously void your warranty.
With 8 months in, you've got a year's warranty, so technically Apple should cover it for free. Green lines with no physical or liquid damage are usually considered a manufacturing defect. What you need to do is go to an Apple Store or contact Apple Support, bring your phone, let them inspect it and see there's no damage. If they see it's clean inside (no water marks, no impact damage), they'll probably replace it for you at no cost.
The only catch is if they find any sign of water or impact damage, even if it's minimal. Sometimes they're pretty strict about it, but if you really haven't dropped it or gotten it wet, you've got a decent shot. Just be prepared to wait a couple days if they need to swap it out for you. Try to go soon because the more time passes, the harder it gets to convince them it's a defect and not wear and tear.