I don't think the octopus is particularly scary, the "Fear / Horror" content warning is intended for "much more" horror, the kind that would get Minetest's age rating increased on Google Play (currently PEGI 12) as content warnings are used to hide such content from the Android version. It's probably a better idea to add that as a note to the description ("May contain octopus, look out in the shipwrecks!") than to add a content warning for it. :)
Maybe the PvE (Player Versus Environment) would be more fitting?
and a warning in the description of the mod
not necessarily phrased as a warning, just being more descriptive of the mod and the features it provides