Step 1: Understand the Open Question Argument.
G. E. Moore’s Open Question Argument posits that for any proposed definition of "good" (e.g., pleasure, will of God), it is always an open and meaningful question to ask, “But is that really good?” Hence, "good" is indefinable.
Step 2: Identify the meta-ethical positions under challenge.
Ethical Naturalism (e.g., defining good as pleasure) is directly challenged.
Ethical Supernaturalism (e.g., defining good as the will of God) is also challenged, since it too tries to define "good" in terms of something else.
Thus, Moore's argument challenges both naturalistic and supernaturalistic accounts of morality.