It was definitely a signal, it was definitely light, and it was definitely aimed at a receiver on the moon. Result is too fast and dramatic to be anything else.
Dunno I'd care to be quite so definite...
... could be coherent X-Ray radiation, if we hypothesize the trail is atmospheric ionisation.
It's good that you're covering all the possibilities. It could be, but I'd just call that high-energy light, for the purposes of this discussion, since (see below) there's a high probability this is a message, not a weapon.
... supposing the target was something in orbit, and the appearance in front of the moon is because the transit across the moon was used for targeting. That would mean the night walker has been looking at the moon in order to monitor transits, which explains why it didn't turn up earlier - had to wait for a transit.
That's a stretch, but I'd buy it if I had to. The appearance of the bright red beacon apparently on the moon could just be something in between.
... and if we assume an object in LEO transiting the moon then the scale, physical and time, comes down to something more reasonable, and we can even consider missile or particle beam rather than light, provided its something capable of producing the ionisation. But that might be propellant of some kind...
Yeah, again possible but a stretch. Why would the moon need to be behind this LEO object before it could communicate? Why not just take Occam's advice and put the object on the Moon?
Loads of other amusing speculations possible, and we have a month to play with them... Consider what in LEO the NW might want to take out...
Well, all the 'Spacers' are in earth orbit, though not necessarily low orbit. But the spacers were put there by something way more powerful, at the Blink. Let's call that something the Eschaton, after Stross's similar scenario. The nightwalker seems to have been around since the Blink as well, so it's not part of some recent Spacer scheme. Anything that powerful on Earth was destroyed at Blink time, so it's got to be something left by the Eschaton.
I think whatever was in LEO at the time of the Blink was either destroyed by the Eschaton along with all the other Earth technology, or left there deliberately for its own purposes. If the Nightwalker is an Eschaton agent (or autobot), it's not going to need to destroy any more satellites now. It's going to send a message.
One thing nobody has mentioned, so I thought I would -- that red beacon on (or in front of) the Moon could also be something physical coming this way.