Praise be to Allah, and peace and blessings be upon the Messenger of Allah. To proceed:
A Muslim should not say: “O Face of Allah,” and this expression is not appropriate in supplication or when calling upon Allah.
Some people who say this may intend by it the Essence of Allah, Glorified and Exalted be He, meaning: “O Allah.” Nevertheless, it is better to avoid this expression and to use clear and legislated expressions when supplicating to Allah, such as: “O Allah” or “O Lord”, and similar expressions.
The expression “O Face of Allah” should not be used. Even if some people intend by it the Essence of Allah Most High, it is better to replace it with a clear and explicit form of calling upon Allah, such as: “O Allah” and “O Lord.”
And Allah knows best.