EXEC UPDATE_STATUS NC_UNIQUEID
That parameter value needs to either be an integer or a integer typed variable. that is why you're getting the error. but in any case you need to be able to deal with multiple values being updated in a single statement.
use this as your trigger body instead:
Update p SET
NC_Status_Updated = getdate()
from inserted i
join ncos_Plain p on p.NC_UniqueID = i.NC_UniqueID
Be One with the Optimizer