The critical SMART IDs are 1, 3, 5, 7 and 10.
From what you show it could be ID 7 that is the problem.
Now, assuming you have a RAID 1 array, I would advise you to power down the unit, take out the relevant drive, put it in a USB/SATA caddy or adapter and run the Hitachi disk tools to test the drive and run a full SMART test. This will determine the state of the disk and if you should replace it.
If it passes all the disk tools tests you can replace it in the NAS and power it on again. Hopefully this will clear the problem but you should keep an eye on that drive for a couple of months.