How to remove or rename a folder with a "bad" name on El Capitan?
Using the bash shell, you can try the completion feature. Example

Ravens-Claw:tmp garex$ mkdir "this\ is\ a\ bad\ folder\ with\ äü\ encoding"
Ravens-Claw:tmp garex$ ls -ltr | tail -1
drwxr-xr-x  2 garex  wheel   68 13 Jun 19:17 this\ is\ a\ bad\ folder\ with\ äü\ encoding
Ravens-Claw:tmp garex$ rm -rf "this\\ is\\ a\\ bad\\ folder\\ with\\ äü\\ encoding"/


Just type rm -rf "this, hit TAB key and bash will do the rest. Don't forget to close the ".

For files with strange filenames, you can try:

rm -- "&lt;start typing filename and hit TAB key&gt;"