There’s no option for stopping and reversing the encryption process. It doesn’t matter that it is an external drive, as it is still the same processs as is used when enabling FileVault on internal drives.
Note that if you unmount/unplug
the drive, encryption will be paused - but it resumes the next time it is plugged in.
In regards to accessing the content - you should be able to do this while the disk is encrypting. If you can not locate the disk in Finder, open Disk Utility to mount the external drive. You can use the drive as normally while it is encrypting - although performance will be impacted a bit.