Dear Herley…
You’re pitching a novel, so those Mangas are a nonissue. They sell into a different market, to a different readership; they’re irrelevant to your novel’s reception or performance in its own market. If you feel compelled to show that you’ve done your research, you could include a line like, “The only books with a similar concept are Mangas,” but I think that muddies the query letter with unnecessary information. Focus on the fact that your project is fresh in your market, because that’s nothing to yuck upon.
Happy writing!
The Editor