It may look like a green Damage marker but Tainted is not damage and should not be confused with damage.
Taunted is a Poison condition that causes "Life -3". For most purposes this is similar to "3 Damage" as a creature with 6 health is equally dead whether it's taken 6 damage, or 2 Tainted tokens, or 1 Tainted and 3 Damage. However, they are healed in different ways.
If you have the ability to remove Poison conditions (Purify, Healing wand, etc.) you can remove Tainted. When this happens, your Life goes back to normal and you haven't taken any damage. This costs 3 mana per Tainted token so the mana-efficiency is similar to healing damage.