He does not remember getting the Echo that allows him to recognize the old ghost; that one had occurred during those lost days before he had contacted the network for the first time. But that ghost is Watari, the one who had looked after Near.

"Possibly. But the slime is the more anomalous of things that have appeared; most have stayed unless they'd been actively removed."

That doesn't make things any more or less worrisome, does it? The ghosts might remain until some way is found to get rid of them. Or they might vanish in a few days' time. Either one seems equally possible to Nathan at the moment.