Alright, so I'm to understand alien nests giving xenomass after the aliens are friendly with you is an intended feature, which makes sense, but the way the game seems to add it infinitely does not. It seems that destroying the nests does not subtract the xenomass after, and that waiting for the nests to respawn or improving them adds the xenomass a second time (or possibly more times, I'm honestly not sure at this point).
See below:
Actual amount of xenomass I have in my territory: 39 (37 improved by wells, 2 improved by a nest at time of the screenshot).
Amount the game is telling me I have: 115
Amount I think I had before I took brutal efficiency policy to try to break the game a bit harder (I forgot to check the turn before I took the policy, might have been more): 94
This game I had a bunch of nests pop up in my territory while the aliens were blue right before I researched the tech that gave me access to xenomass wells. Needless to say I then walked on them and put down my own improvements which is where I noticed this. Further, I seem to recall a nest giving me extra xenomass a second time after respawning in my territory after I destroyed it the first time but didn't improve the tile. I'll try to confirm that in another game later when I get the chance. In theory this means infinite xenomass by destroying nests and letting them respawn over and over, but again, I haven't tried to confirm this yet. I will try to confirm soon.
I can post the save if need be. Or if someone wants to build a trillion xenotitans and run rampant. Whichever.
Edit: Oh right, and it seems that tiles that had a nest on them prior to me being friendly with the aliens (ie: the nests I acquired to make them friendly with me in the first place) do not give xenomass at all. They needed to be destroyed and allowed to respawn/improved for the xenomass to show up.
See below:
Spoiler :

Actual amount of xenomass I have in my territory: 39 (37 improved by wells, 2 improved by a nest at time of the screenshot).
Amount the game is telling me I have: 115
Amount I think I had before I took brutal efficiency policy to try to break the game a bit harder (I forgot to check the turn before I took the policy, might have been more): 94
This game I had a bunch of nests pop up in my territory while the aliens were blue right before I researched the tech that gave me access to xenomass wells. Needless to say I then walked on them and put down my own improvements which is where I noticed this. Further, I seem to recall a nest giving me extra xenomass a second time after respawning in my territory after I destroyed it the first time but didn't improve the tile. I'll try to confirm that in another game later when I get the chance. In theory this means infinite xenomass by destroying nests and letting them respawn over and over, but again, I haven't tried to confirm this yet. I will try to confirm soon.
I can post the save if need be. Or if someone wants to build a trillion xenotitans and run rampant. Whichever.
Edit: Oh right, and it seems that tiles that had a nest on them prior to me being friendly with the aliens (ie: the nests I acquired to make them friendly with me in the first place) do not give xenomass at all. They needed to be destroyed and allowed to respawn/improved for the xenomass to show up.