I tried adding this to an existing save and there weren't any sludge veins, even when I explored out of my bubble. I then regenerated the terrain using this mod but I still couldn't find any. It could just be bad luck but I walked several miles and didn't see anything.

I wonder if the resource richness and frequency defaulted to zero when added to a game? Or maybe you need to listen to some script event when being added to a save or regenerating chunks.

Sludge stain are rare, spawn a while from 0.0 and I am not sure if you don't need to regenerate surface normally

I never tried this mod.
I will see if it works and let you know

Just tried it out
Using this mod does not regenerate resource
I was able to find holmium sludge only after:
hoping in editor mode
removing surface
generating new

i will see if I can solve it in some other way

Okay
I think it should now work using migration

Can you confirm that everything is fine?

That worked. Thank you!