What kind of files (7f7cc4.css, 1e13a6.) and how to clean them?

Noticed that when you enable the file merge feature in folder /media/mj/ to appear files of the form: 1e13a6.js and 1e13a6.js.gz
, 069ece.css, 0bcc69.js. This folder is self-cleaning and through what time or should it be done manually?

Yes, it's self-cleaning (daily).