I have seen a comment left from a moderator about a question being asked; I agree with what the moderator said about the question being off-topic. Do I need to do anything, or will the moderator take care of the question?
Sometimes moderators leave comments saying what it is wrong with a question in its actual form; not all the times a moderator then takes action about that post.
As a matter of questions to close, moderators close questions that are blatantly duplicates, off-topic, not real questions, or not constructive questions. If you happen to agree, you can vote to close the question too, or flag it choosing "it doesn't belong here." If the question is not so very obviously a not desired question because it's a borderline question, moderators will probably not take any action, and it will be the community that needs to close the question.
This is also true for comments left from moderators about answers: If a moderator leave a comment on an answer, it doesn't necessarily mean the answer will be deleted. If you think the answer is a really bad one, which is not useful at all, you can down-vote it.