Can I get Nahkriin's mask if I've already left Skuldafn?

Nahkriin is found in Skuldafn, therefore if you have completed The World-Eater's Eyrie from the main storyline then you will not be able to acquire the mask without console commands.

If you are on PC, you can use the following console command to add it:

player.additem 00061CA5 1

However if you are playing on consoles then you are unfortunately out of luck for this play through unless you have a save before completing the main story line.

There is one way if you go back to the save (if you have) at any point in the game and it will save after you have killed the priest but quickly go back to where you where originally (you have to save before you go back to the mission then once you have got the mask go back to the save you did were you where at at any point in the game) then if it works (its kind of a glitch) you will then have completed the task and should have that mask. Hope this helped

For the XBox One there is another possibility: The Mod "The Cheat Room".

This provides, amongst other things, access to every item in the game. It is, essentially, the back-door mechanism for "using the console" when you can't use the console.

There is, for the PS4, a similar but different mod provided by TheRealElianora (who provides the House Creation Club entries). It provides access to the original test cell which already has "all the items. This one is not as powerful as the XBox version, but it would provide this item. The mod is called "PS4 Cheat Mod". There is also an XBox One version of this mod.