So, I was thinking. What's the reason for Pale King taking half of Greenpath? He doesn't seem the type to just take land because he can, and surely it's not just for White Lady to have a pretty retreat.
So, my guess is that it's farmland. Think about it. You're building a kingdom with all these bugs you've just granted intellegence, and they need something to eat. But what? Sure, you can eat animals (crawlids, tiktiks, gruzzers, etc), but another option (perphaps, an easier one, or just the one that was more convenient at that time) is just to grow food.
Greenpath seems to be the only fertile land in Hallownest (sure, there is some vegetation in Kingdom's Edge, but not a lot). And since the Mosskin probably aren't very interested in becoming a food exporter and/or it's more safe/convenient for your civilisation to just do it yourself, you make a deal with Unn for half of her land (with some additional small terms) and in return Mosskin get left alone to do their own thing