A moderator edited my post by removing what I thought was a relevant tag. Is there a way I can ask them directly why they did that?
Yes. Ask them as a comment to your question. I would hope that they would have commented when they edited...
Posting a question here with a link to the question is also a good way to discuss with others why the change was made.
I'm assuming you're talking about this change:
It is likely that the reason why
iphone-ui was removed while
ipad-ui was left alone was because
iphone-ui is a new tag to stackoverflow, and the person who removed it did not feel it was necessary to add another tag. The editor who changed it probably keeps an eye on newly created tags and removes those that seem unnecessary.
Personally, I would rather have it tagged
ui rather than combining them primarily because we really don't need more specialized tags, and while the iOS user interface is a little unique, there are plenty of mobile devices which share similar ui design rules.
If it was an actual diamond mod, you could flag that post with your query. But if if was just a regular editor/tagger you will need to use the @<username> comment format to address them.