How about you give us some more details or a list of your mods?

Disable all mods then enable them one by one, see when it appears. You could also speed up the process by starting with all mods disabled, check, then enable half of the mods, check, then enable another half (of those still disabled), check, etc. Unless something is screwy, you should be able to narrow it down quite quickly.

I'd also start a fresh mission or campaign to test with, don't use an old save that had the green gas thing.

Sounds like an alien vent releasing toxic gas, if so this is no mod but probably a custom sub build with malicious intent.