Can't delete iMovie library files from MyCloud EX2 Ultra

I am trying to delete some files from my MyCloud EX2 Ultra that include some iMovie projects. I find I cannot delete any iMovie library packages (.imovielibrary), as it says various files are in use (usually .fcpcache but sometimes also something like “people finder” or folders like “Original Media”). Needless to say, none of these projects are in used.

I am using a MacBook Pro running Mojave (10.14.6) to manage the deletions, if that matters.

This is extremely frustrating as these files take up a lot of space, and I’d appreciate any advice. Thanks!