Yes, but Cerys is a moon of Fulgora...

Hi, yeah, Cerys is a moon of Fulgora, but I replaced its home planet, Fulgora, and placed it in orbit around a gas giant.

so you need to fix it in planet description i guess, because it`s still moon of fulgora)

To change a planet's description in another mod, you have to directly modify the global settings—something only the mod's developer can do. It's a shame, but that's just how it is.

To change a planet's description in another mod, you have to directly modify the global settings

This statement is wrong on at least two levels:

1. Planet descriptions that appear in-game are contained in the respective language's locale file (check the locale folder to see the various folders containing cfg files); this is not related to "global settings" in any way
2. Other mods CAN override these in-game descriptions from within a third-party mod (that is basically what Rubia Alt Cut does)

You can very easily change the planet descriptions from within this mod