I am noticed that one strange folder name on my hosting.
It is an empty folder.
After some search, I found it is a temporary folder which is used to test server connection and writing permission.
MM_CASETEST4291 is created by Dreamweaver. It is designed to create a folder, then delete it.
So I remove this folder by commend
rm -r MM_CASETEST4291
No hacking, no pain. Safe.
You can also doing some search by Google to see the indexed empty MM_CASETEST4291 folder. Follow the parent path, you can find some interesting thing.