I admittedly have an extensive mod list (mostly razi protocol things) which could be causing this, but I wanted to check if its just the mod itself. Cerys seems to have pollution enabled on the planet, despite the fact that without enemies the pollution doesnt do anything gameplay wise. Meanwhile, its technically got a UPS cost to be doing pollution calculations, presumably inflating the map size based on how many chunks my cloud explores into the space around cerys, and its making my beautiful moon water all nasty and green. How can there be pollution on a moon with no atmosphere?

Cerys has pollution turned off already. Are you sure the water is changing due to pollution? Water on Cerys is naturally greener than Nauvis water. Could you provide a screenshot?

Aww, never saw your reply. I think you should be able to view this steam screenshot:
https://steamcommunity.com/sharedfiles/filedetails/?id=3797271450

With that said, if you're saying pollution is properly turned off on the cerys base mod, then its probably either k2so's fault or something else in the razi pack doing it (probably the same thing that added enemies and pollution in hyarion). I'll keep digging.

After digging around, I think its probably related to razi protocol explicitly adding frost biters to cerys. And because its such a small planet, no frost biters ever spawned (according to my production/kills graph). So its just wasting ups. Lovely.
https://github.com/Raziphel/razi-protocol/blob/main/prototypes/compat/cold_biters.lua line 33 and 115
Anyway, its not your fault. I'll go complain to razi.