Green nail syndrome is when your nails turn green because of tiny creatures called bacteria that like to live on the nails. Just like how germs can make you sick if they get into your body, these bacteria can make your nails look sick if they get into your nails.
Your nails are made up of layers, and the bacteria can get in between those layers and start to grow. They like to eat things that are on your nails, like dead skin or leftover nail polish. When they grow, they make a green pigment that shows up on your nails.
Having green nails is usually not harmful or painful, but it doesn't look very nice. You can prevent getting green nail syndrome by keeping your nails clean and dry, not leaving nail polish on for too long, and not biting your nails. If you do get green nail syndrome, a doctor or nail specialist can help you get rid of it by cleaning your nails and possibly giving you medicine.