When a tag has no questions related, or misspelled, or is not about physics, what can we do about it? Is there a way to flag it? Or to ask for deletion?
EDIT: the mechancis tag seems to have been removed.
If a tag is misspelled or otherwise inappropriate (e.g. not about physics) and is only on one question, just suggest an edit that changes the tag to the correct spelling, or removes it, or whatever.
If an inappropriate tag is on two or three questions, you can just edit them all accordingly.
If a misspelled tag, or a tag which needs to be changed (not removed) for some other reason is on a lot of questions, there is a process by which you can suggest a tag synonym, and the community can vote on it. But that process is fairly well hidden. So in practice, the easiest thing to do is flag one of the questions for mod attention with a custom message.
If a tag needs to be removed outright from a large number of questions, without any replacement, that needs mod attention. So definitely flag in that case.
As regards the quant-ph tag, I see a link with a MathOverflow policy which asks all users to tag their questions using at least one arXiv subject area. While we don't have any such policy, I suspect some MathOverflow user applied the tag once, and it stuck.