I was able to get Graring to hand over the Ancient Nordic Pickaxe, but when I attempt to use it on Stalrihm, it just swings through without effect like any other weapon. Any suggestions as to why this is not working? How to cure it?

I haven't done that quest in a while, but I don't think you actually swing the pick at the block of stalhrim (though that'd be cool if you do or if I'm wrong). I think you just activate it with the pick in your inventory.