MC is not critical to Stark, so you can let Grey take MC and not care at all. Actually, apart from Shivering Sea and Narrow Sea, no territory is critical to Stark, even losing Winterfell is quite common but with a good S in Shivering, you can usually retake sooner or later.

Stark's keyword is "resilience". With the biggest natural lands, your troops can pretty much always retreat safely, while inflicting lots of damage with a properly placed Ned or Robb that you'll get back with Roose.

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Judge
LoI&F: Seasons 10 and 11 King
France WCup team
2021: Bronze
2022: Silver

With no mustering, his troop should be unsupported and easily attackable.  You want him out as soon as possible, but it should be very doable to kill the troop, and again without mustering, it's a troop GJ can't very well afford to lose.

We actually have such an example in current World Cup, with that game :

https://game.thronemaster.net/?game=309602

Greyjoy can look in a good position with 5 castles, but he is actually in a lot of trouble : little troops, no sea control, bad card management (no Balon), little CP generation. If the game goes to turn 10 he'll probably get wiped out or end at least in a very bad spot. If you look better, the main issue for Stark is not Greyjoy, it's Baratheon who seized its seas.

That's actually the main issue in the current meta : most Greyjoy go north first ("Stark left alone is a winning Stark" sums up well the Northern position) to put some pressure on Stark. If Stark consumes too much resource on its western coast and fails defending on the eastern side, he is doomed. On the other side, if he holds well east, he should in the long run be resilient enough to resist to Grey (better CP generation, Eyrie as backup capital, ship support in SS...).

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Judge
LoI&F: Seasons 10 and 11 King
France WCup team
2021: Bronze
2022: Silver