Noticed that there is a configuration file named “moss_patch_bonemeal.json” in vanilla worldgen datapack, placed in "worldgen/configured_feature". However, modifying this file does not change the spreading behaviour of moss block, but modifying “moss_patch.json” can change the moss patches generated in lush caves.

A datapack for testing is in attachment. 

I also checked the code and confirmed that the spreading of moss block uses configured feature to generate moss patches.

//net.minecraft.world.level.block.MossBlock#performBonemeal
@Override    
public void performBonemeal(ServerLevel serverLevel, Random random, BlockPos blockPos, BlockState blockState) {
    CaveFeatures.MOSS_PATCH_BONEMEAL.place(serverLevel, serverLevel.getChunkSource().getGenerator(), random, blockPos.above());
}