First, take it easy guys! There is nothing personal in my question. I've asked this to Alex Gruber directly few months ago, and he told me that his lack of activity on M.SE is due to research and moderation (!!!). Unless he's doing some invisible job (which I doubt because he doesn't visit M.SE very often), I think a honest move form his part would be to quit. Some other people already did this, so won't be a novelty for M.SE, and everyone will thank him for what he did so far.
IMHO a moderator should visit regularly this site (to not say very often or daily, as most of them do), and help others by being active in the answering process. If you check his profile, his last answer has been posted on November 6, 2015, that is, almost a year ago. There are many users who can replace him successfully, but it seems the moderators are here like academicians, once they are elected no one can replace them. This is not normal on such a dynamical site, and I hope my post can make him to think a little about all of these. He is also invited to comment/answer my question.
Edit. After one month and a half since I've asked this infamous question our dear moderator Alex Gruber is still missing.