Is “Grey tsunami” a mere metaphor, or acknowledged phrase to represent for accelerated global increase of old age population?

The easiest thing to do is look for other uses, and if you do so you'll see that it is indeed used. Note though that while there are other uses, there aren't a lot. Silver tsunami is more commonly used of the same thing, though still not very common. It's perhaps best thought of as a minor buzz-phrase that a lot of people will use, and those who don't know will understand due to the metaphor being clear, but not something everyone will have in their productive vocabulary. (Or for that matter, be happy with if they do know it, there are objections to the metaphor, not least that an unpredictable event like a tidal wave* is not an appropriate metaphor for a demographic shift that's been easily foreseeable for decades).

It also appears to be a largely American use, though other English-speaking countries have similar issues (though not all countries do, or do to the same extent).

It refers primarily to the effect on the employment patterns, social security and health of a given economy rather than just the numbers themselves. (A rather old-fashioned use of quantum in most dialects, incidentally, though still popular in that sense in Indian English).

*Incidentally, whether figuratively or literally, poor old "tidal wave" is a phrase that got an unfair bad rap just because tidal waves have nothing to do with tidal activity, and so the loan-word tsunami has all but entirely replaced it. Really, since tidal waves are indeed waves that look like an incredibly low then high tides of great suddenness the term is perfectly apt, while they are in no way restricted to harbours making tsunami the choice that has the weaker connection to the reality. The only thing that makes tsunami seem more accurate is that most English-speakers don't know what tsunami's components mean.

The use of "tsunami" as a metaphor increased after the Indonesian/Indian Ocean tsunami of 2004, possibly because this previously generally less familiar word now had many more people who understood it. A quick check online now indicates that "silver tsunami" and "gray tsunami" (both essentially a more striking way - a 'colourful phrase' - to say "ageing population") has been in use since at least 2010 - search for comment by novelist Martin Amis (in 2010) and article by Philip Longman (in 2010). See Economist article from February 2010: http://www.economist.com/node/15450864