This expression is completely fictional. In the Harry Potter books, there are other 'wizarding idioms' that sometimes paraphrase ones existing in the real world. A (presumably, incomplete) list can be found here.
It is perfectly acceptable to use them in the Harry Potter fandom. Certain expressions from the books are well-known to this community, and you won't raise any eyebrows by saying "Would you care for a lemon drop?", "Expelliarmus!" or "Constant vigilance!" even in a seemingly unrelated situation.
You can easily remember similar expressions from other books or movies: "May the Force be with you!" from the Star Wars, "I'll be back" from the Terminator. Still, you cannot assume that every speaker of English knows each one of these (keeping in mind the amount of available books, movies, cartoons and comics).
So, my conclusion is that "Merlin's beard" can certainly be used as a personal catch phrase in the Muggle world. Just don't expect everyone to recognise it.