Question: what colour are little fingers eyes?
Answer: The prophecy may also refer to Littlefinger (a.k.a. Petyr Baelish), whom Arya already killed in Season 7. He's said to have gray-green eyes in the books. (In real life, however, actor Aiden Gillen has blue eyes.) Maybe Arya has already fulfilled her prophecy and sealed the deal with the Night King as her victim.