Is that why you are using docker folder instead of docker.img? You shouldn't need to access anything within docker.img.
Try recreating (as 20G image, not folder) and reinstalling
https://docs.unraid.net/unraid-os/manual/docker-management/#re-create-the-docker-image-file
But before that, your domains and system shares have files on the array. Ideally, appdata, domains, and system shares would have all files on fast pool such as cache, with nothing on the array, so Dockers/VMs will perform better, and so array disks can spin down since these files are always open.
Nothing can move or delete open files, so you will have to disable Docker and VM Manager in Settings before you can work with these. Since you have Dynamix File Manager plugin might be simpler to clean this up yourself instead of trying to reconfigure so Mover will take care of it, and Mover won't replace files anyway so if there are duplicates you will have to decide which to keep.