An "A" For Our Favorite PLL English Teacher, Ezra Fitz: Based on PLL 4x14, Who's In The Box?
Mona confronts Hanna about the girls shutting her out AKA keeping secrets from her. Later in the episode, we see Mona approach Ezra and strongly insinuate that she knows about his connection to Alison and/or "A." Mona has hardly spoken to Ezra before, so this is an interesting time for her to start. If my long-time theory about Mona and Alison working together is correct, it would explain Mona's sudden interaction with Ezra. Alison is seen in the PLL promo telling Emily that she thought she knew who was after her, but she was wrong. Alison could have told Mona the same thing, which could have led Mona to begin sleuthing to find out who Alison REALLY fears the most. IF Ezra is indeed the person who is scaring Alison, perhaps Mona has started to figure out exactly what he's doing. "A" or not, we know that Mona has the capability of spying, lying, manipulating ETC.
She mentioned Spencer, Emily and Aria when talking to Ezra. Perhaps she did this to dangle in front of his face the fact that she knows that Ezra is spying on the girls to find Alison. She emphasizes that he's this GREAT teacher/advisor/confidante ... by day. Then she discusses her interest in Jekyll and Hyde, a story involving someone with a kind personality and a secret dark, dangerous one. Mona could be figuring this out about Ezra. It would also be an interesting parallel to when Mona was the lead in the play, "The Bad Seed," that Ezra was in charge of, and she made a point to mention that she couldn't tell if her character was evil or just naughty. This was back when Mona was still "A," meaning that she was spying on the girls. Since that includes spying on Aria, it is very possible that Mona found out information about Ezra during her time as "A." Maybe something she found back then is triggering her to suspect Ezra now.
Why else would Mona arbitrarily start a conversation with "Mr. Fitz" as she called him? She's mad that the girls are shutting her out. Shutting her out of what exactly? She could have spied on them with her surveillance tool of choice on the night of the PLL/Ravenswood special. She could have seen the girls find Alison alive... and she could have seen that Ezra was following them.
If she saw that and is indeed working for Alison, this is the perfect scenario for Mona. Ezra doesn't trust Mona; he knows that she was the first "A" and was torturing the girls. He mentions it in the conversation they have in this episode. Ezra doesn't think that Mona cares about any of the girls, Alison included, so it's unlikely for him to suspect that Mona is onto him and his desire to find Alison. This makes it perfect for Mona to start digging, and she could be using Jekyll and Hyde as a tool to start getting information.
Additionally, if Mona is working with Alison and saw Ezra following the girls, and perhaps even saw Grunwald telling the girls that a guy is following them in order to get to Alison, it's likely that Mona HAS figured out what Ezra is up to. If Mona knew that someone is after Alison, and Mona has been following the girls, it'd be easy for a smart girl like her to put two and two together. Under this theory, Alison has the perfect person doing her dirty work for her: someone who nobody would suspect; somebody who couldn't stand her; and therefore someone who is the most likely to get away with getting Alison all of the information that she needs. Ezra told Mona that fear is a great motivator; perhaps Alison's fear of Ezra motivated Mona to investigate Ezra.
At the end of the conversation when Mona seemed intimidated, it could be because she took Ezra's comment about how far someone could be pushed before they get pushed back, as a threat. And perhaps his comment DID have a double meaning; maybe that's how he felt about Alison, but the words still were surprising for Mona to hear since she doesn't think Ezra would suspect her of helping Ali or the other girls. Lastly, I mentioned above that Mona referred to Ezra as "Mr. Fitz." Of course, this is normal behavior, as Mona is Ezra's student, but I also think it could serve as more emphasis that Ezra is unlikely to guess that Mona's conversation with him has anything to do with the other PLLs (who all know him as Ezra)... and that Mona knows this and is using it to her advantage... and to Alison's advantage.