I recently saw an unfriendly comment from a diamond moderator on an SE site. I would flag the comment as unfriendly or unkind if a regular user had posted it. How should I respond considering that it's from a mod?
(This is unrelated to the strike.)
Flag it anyway!
On the sites I moderate, if something posted by a moderator gets flagged, that mod will recuse themself from handling it (and in fact will usually ping the rest in a private chat room to get as many other mod eyes on it as possible).
If a single mod seems to be abusing their power by wrongly declining all flags against them, you can bring it up on Meta (with specific examples of the inappropriate content): either the per-site meta, or here if necessary.