Could your story be low concept?
I would definitely not pitch it that way, as it may come across as "not exciting" -- or, indeed, hard to explain, which can equal hard to pitch or shelve, which can equal No.
I don't think this is as hard to explain as it might appear. It's a middle-grade ghost story. When you write your query and present character, problem/goal, action, complication, and stakes, this will *show* that it is a ghost-hunting story rather than, say, one where the characters find out that the place they're in is haunted. You don't have to try to explain that it's a ghost hunting story in exactly those words.
What concerns me is that you may not have a child front and center. I could be wrong, but I pictured your ghost hunter as an adult. If you can't make the ghost hunter a child, maybe give him/her a child "sidekick" who is actually the MC.
I second the advice to talk to a librarian AND bookseller to get the best of both opinions and find your comp titles. According to recent experience and industry talk, comps and how you choose them are becoming more and more make-it-or-break-it to your query or pitch.