Art by Lynn Phillips Nelson
We all know the first line of a query is your "hook." I call the last line the "sinker." You want it to punch them in the face, in a nice, friendly kind of way that makes them unable to forget you after having read the 300 other queries in their inbox.
Also, for my brave Saturday Slash volunteers I will gladly do follow-up slashes (each more kindly than the next) on your query if you post them on the Query Critique board over on AgentQuery Connect. You'll get advice from me, and also people who are smarter than me. If you do post on AQ, be sure to follow the guidelines and let me know you posted so that I can follow up!
Now on assignment for the first time, Mason knows what he has to do: find the Dreamer wait - what Dreamer? Surely not the one whose body parts he got in a box six years ago? Because that wouldn't quite make sense..., save the world, become a hero. Easy, right? Sure, until he meets seventeen-year-old Avery Carmichael, who, according to his records, should be a boy. Nice - I like the twist here.
Avery makes Mason’s job anything but simple. Haunted by her past, she lives in fear that the people who took her parents will come for her next. So the body parts were / was one of her parents? So when Mason shows up and tells her she’s not crazy I'd strike this , that her dreams really are coming to life, she’s not sure what to believe. But her inability to control her gift brings the Dream Catchers closer and the duo must learn to work together—despite their growing feeling why would that be a block? I would think growing feelings for each other would make it easier to work together—to avoid them. If this group catches a Dreamer like Avery, or as legends call her a Chavez—one who can completely control their dreams— I'd strike the information here after "Avery" it's making everything choppy. I like the idea of a "living nightmare," great sinker they’ll use her power to change the world into a living nightmare.
Narrated alternately by Mason and Avery, DREAM MAKER, a YA speculative fiction novel, is complete at 60,000 words and on multiple submission. I wouldn't worry about specifying multiple submission - they assume that.
Overall this is looking pretty good. You're trying overly hard to get your world building jammed into parenthetical phrases here and it's not absolutely necessary. Cut out some of your excess and get that body box into the first line and you're looking much better.