Green mutagen is an alchemy ingredient and is used to craft Greater green mutagens. They can also be used to augment abilities, but are not as effective as their greater counterparts. When placed in the diamond-shaped slot in the character panel, they give +100 to vitality, which increases by 100 up to a max of +400 if all 3 skills attached to it are green.
Blood and Wine expansion
With Blood and Wine, the player can now craft these with other ingredients that don't include lesser green mutagens.