'v5s39n' doesn't appear to be a plugin from the directory or a publicly available plugin. Maybe it's a bespoke plugin for your site.
I'd recommend investigating how it got there. Check any timestamps on the mod/v5s39n folder, and on the files and subfolders within it as these should reveal when this was created giving you a clue as to how it got there. The contents of any files should give you a clue as to what the plugin was designed to do.
There's the possibility that mod/v5s39n was created during some attempt to attack or breach the security of the server. Hopefully your investigations above will allow you to determine whether its provenance is benign or otherwise.