Are you using WD Sync on computers to sync content to the My Cloud?
If so there is a well discussed problem (see the forum search feature, magnifying glass icon upper right) in many past discussion of WD Sync filling up one’s hard drive. WD Sync creates a hidden folder that stores versions of synced files. It allows the program to restore a file if the user deletes it by accident.
WD does not offer any sort of “cloud” based storage to go along with the single bay/single drive My Cloud.
You can add a USB hard drive to one’s My Cloud to expand the My Cloud storage. The USB drive shows up as a new Share when connected to the My Cloud. The USB drive can also be used to backup the My Cloud drive using Safepont or Backup depending on which version My Cloud firmware one is running.
Or you can go with any other online cloud storage service both free and paid if you so desire. You would have use that services apps/software to move content from your mobile device or computer to that online storage service.