Sadly its not possible to add this mod to an existing world.

Maybe you could add a tool or a script that replaces vanilla cliffs whith yours when adding this mod.

Do you have some other mod that manipulates the cliff prototype?

The mod has an on_init that searches for all existing cliffs and creates the mineable prototype layered on top of the existing cliffs when the mod is installed or enabled on an existing world. When the mod is added to a save, it searches for cliffs, adds the player interactable version, and when the mod is disabled, all of the mineable cliffs are thrown away. This makes it safe to add and/or remove on existing saves without risk of destroying the existing cliffs on the map.

If you have some other mod that manipulates cliffs, I can certainly look into generalizing the search for existing cliffs, or allowing you to manually specify prototype names. I need more information from you though. What is your modlist? Which of them modify cliffs?

I got some free time and looked into this. I believe I have isolated the issue. I'll see about a generalized fix in the next few days.

For clarification, the issue appears to be with what alien biomes is doing with world generation.

FYI I have Alien Biomes and this mod works just fine for me when added to an existing save.