The Yaksha can jump the full 6" but he has to arrive in base contact with the Moorwraith:
Other friendly seeker models may treat this model as a severe impassible terrain marker
The Yaksha is a friendly seeker, so fall in this category
Other models treat terrain area within 2 of this model as Severe
You are in the 1st category, so not "Other models"
Also, base on this document (not it's not an official FAQ), there is a point about
https://docs.google.com/document/d/1Z6jmZUtGXBxx31gHywREs95h-26js4maH0JBsBLWRRU/edit
Now that explanation is odd to me, but I prefer to have a consensus rather than nothing and argue during the game ...
So, taking that point in, I think it would apply the same here, the Yaksha need to be in base contact, both for the heal and the jump
So, only in base contact with the model (Kurgan or Moorwraith)
I'm waiting after Wyrd to officially clarify this point of rule 🙏