I mean that if I think I am "master" in some topic or tag (iOS address book, localization etc.), I wish to get an alert when a user posted a question tagged with a specific tag. Is there a way?
You can do that by subscribing to the tags you are interested in, thus getting email alerts – you can subscribe from the tag mouseover.
However, if you're familiar with RSS, I recommend to use the builtin RSS feeds, since you can do that not only for single tags, but also for some boolean combinations of them: see this blog.SO post.
Another way is to feed the topics to a slack channel.
Suppose you create a new team in slack
After you login the slack team, go to one channel and run below command to add a new topic
linux in it.
/feed subscribe http://stackoverflow.com/feeds/tag?tagnames=linux&sort=newest
Then you will keep getting new questions about Linux from stackoverflow.
You are fine to add any topics which you are interesting with below format:
/feed help to get help.