The only reason I say Julian is because when Franco opens the hotel door, he recognizes the person with a surprised expression. So i'd guess Julian, Heather, Morgan or even Ellie (to check on him after his suicide attempt). These would be the people he'd be surprised to see.
(I'm guessing whoever is lurking is the same person that shows up at Franco room at the Metrocourt)....just guessing...so that would exclude Morgan and Ellie.
Yeah, not Jason's ghost (that's would piss off the fans), not Cole or Hope because they don't exist in this current GH universe anymore.
While it is being Jason's birthday, perhaps it IS jason. And the producers will leave it at that. Hinting to the fans he's alive, out there, and keeping watch. Mind you, Jason doesn't seem like the lurking creepy stalker type.
As for the most likely stalker/lurker/creeper: I say Heather. Cause she's connected to Sam, the Q's etc. If not heather, then maybe Ava (as she's been keeping a close eye on the Q's, Silas, Sam, Sonny & his organization, Duke & Anna).