What is the authenticity of the widely circulated supplication, "I have veiled myself with the light of Allah's Ancient, Perfect Face...", which is said to protect against envy?

There is no evidence for the authenticity of attributing this supplication to the Messenger of Allah, peace and blessings be upon him, or to any of the Companions or their successors. For beneficial supplications, it is recommended to refer to books of remembrance such as "Hisn al-Muslim," "Al-Adhkar al-Nawawiyyah," and "Amal al-Yawm wa al-Laylah."