I doubt that there is anything to cancel. A disk cannot AFAIK do a long test and a short test simultaneously, and since you have more recent successful short tests, the long test you kicked off 6 days ago is probably no longer running.
I am not sure what happens on a drive if a long test takes more than 24 hours and a short test is requested whilst it is still running, but I would suggest that you temporarily turn off the scheduled tests on this drive and manually run a long test to see what happens.
Also, please do a zpool status -vx to check whether you are getting any ZFS pool errors.