Just using the name "Marcel the Shell" probably isn't enough to be legally actionable. Plenty of books, films, and TV shows have the same titles, which indicat

Just using the name "Marcel the Shell" probably isn't enough to be legally actionable. Plenty of books, films, and TV shows have the same titles, which indicates that titles alone typically don't enjoy the protection of exclusivity. Copyrighted characters and trademarks have a better chance of that, but those aren't being appropriated here, just the name. "Marcel" is also a common first name, which makes it less likely that the creators of the film can claim any exclusive use of it, and "the Shell" is completely descriptive in this context, so unlikely to cause copyright issues.