The directory .debris is not hidden because of the hidden flag, but because it contains . in the front. By default such files and directories are not displayed in GUI (and regular CLI ls command).
You can try just to move the subdirectory 2016-07-22 outside of the hidden directory:
mv -i ./.debris/2016-07-22 ~/2016-07-22
or rename the .debris directory with:
mv -i .debris debris
The error chflags: ./.debris/: No such file or directory is however strange, maybe you were executing chflags from a different directory. To be 100% sure, use absolute paths.
I guess your .debris is in your home directory, so you can use a ~ (tilde):
mv -i ~/.debris/2016-07-22 ~/2016-07-22
or
mv -i ~/.debris ~/debris
The -i flag protects you against overwriting (and losing) files if the destination directory existed. It's not likely, but better stay safe.