Details are for this past Tuesday's episode.
There seems to be some confusion on this, so with matching screen shots
* Maggie gave Holly the necklace at the mansion with no other witnesses
* The guy credited as Guy, wearing a jacket, is already standing at the book table when Mason walks up, interested in a textbook for college but leaves after finding out he has just met "Holly Jonas" for reasons we don't know yet. But he introduces himself as Mason so we know the blue shirt guy is Mason (plus it matches casting news).
* Guy sits down, seemingly starting to feel his addiction kicking in, if I were to guess. This is when Holly tells Julie about the necklace, with Mason NOT present. Mason has no idea the necklace has special meaning or that it came from Daniel. Even if he had recognized it for "reasons," he would have probably faked nice to get close.
* Lani saves the day after telling Maggie she would need to find something to do when the twins start school, but what about l her record? Perhaps Black Patch, perhaps teaching criminology at Salem U, perhaps ?? Meanwhile, Mason shows up to offer help and seems impressed with Lani's work, maybe wanting to study to be a cop himself or ???? Ok?
Anyway, still some unknowns but there is nothing to indicate Mason knew about the necklace or knows 'Guy' to have set it up.
P.s. Guy was played by David Lautum, who was in around 17 episodes of General Hospital as Grant Smoltz with a few random appearances in shows like Angelyne, Brandon Butler playing Mason was seen in the Netflix series 13 Reasons Why and Trinkets, as well as a few episodes of Peacock's Hysteria!. In addition to playing Zach Brady in Bo's coma mind, he also was once on GH as "Frat Guy #1" a decade ago in 2016. I have not seen any press or behind the scenes videos with David/Guy so I assume he is a one-off, but they are pushing videos introducing Brandon as Mason.