It would not be considered colloquial language, however used in context it would certainly show you have a good level of understanding.

It is certainly not the kind of thing that I say every day, but I say it. It is a useful phrase. I wouldn't see any problem with using it in a speaking exam.

It isn't formal. It is, however, a word used by more-educated people. It sounds very natural when used appropriately. It might be dangerous to use it in a speaking exam, because it might sound as if you were trying to show off advanced vocabulary, so be certain that you know its range of meanings and use it correctly. Its most common use is to describe a theoretical issue that doesn't matter any more because it's been overtaken by events. "I just discovered that the old schoolhouse on Hill Street is protected by the State Historical Preservation Act." "Unfortunately, that's a moot point, because the developers demolished it yesterday."