I've been trying to add a couple of new buildings and edit the exterior of Raven Rock but came against am issue I haven't encountered before, I'm unable to create, move or edit vertices/triangles in many areas, and I can't connect vertices together either. When I try to connect or add triangles they snap into the existing navmesh border, and attempting to push it out hits an invisible barrier

I've also noticed these green triangles in the area
http://i.imgur.com/B4bfvoP.jpg

What do they do? do they connect navmeshes together? if so how do I add them? and is there a way to get around the issue I mentioned?