Mooshroom pathfinding favours grass rather than mycelium.

Causes:

Caused by harcoded use of GRASS block instead of instance attribute, spawnableBlock (MCP for 1.9.4):

In entity/passive/EntityAnimal: #20 (EntityAnimal class):

    protected Block spawnableBlock = Blocks.GRASS;

In entity/passive/EntityAnimal: #82 (EntityAnimal class):

    public float getBlockPathWeight(BlockPos pos)
    {
        return this.worldObj.getBlockState(pos.down()).getBlock() == Blocks.GRASS ? 10.0F : this.worldObj.getLightBrightness(pos) - 0.5F;
    }

since MushroomCows redefine their spawnableBlock to mycelium:

In entity/passive/EntityMooshroom: #23 (EntityMooshroom class):

    this.spawnableBlock = Blocks.MYCELIUM;

this have no effect on their pathing behaviour as the perferred block is hardcoded to GRASS.

Solution:

Replace
In entity/passive/EntityAnimal: #84 (EntityAnimal class):

 return this.worldObj.getBlockState(pos.down()).getBlock() == Blocks.GRASS ? 10.0F : this.worldObj.getLightBrightness(pos) - 0.5F;

replace to:

return this.worldObj.getBlockState(pos.down()).getBlock() == this.spawnableBlock ? 10.0F : this.worldObj.getLightBrightness(pos) - 0.5F;

Sideeffects of the current state:

Passive mobs are intended to spawn in light levels of 9 or above, as signified by this expression:

In entity/passive/EntityAnimal: #122 (EntityAnimal class):

return this.worldObj.getBlockState(blockpos.down()).getBlock() == this.spawnableBlock && this.worldObj.getLight(blockpos) > 8 && super.getCanSpawnHere();

However since mooshrooms will path through grass, the following code takes an effect while spawning on mycelium:

In entity/passive/EntityCreature: #40 (EntityCreature class):

        return super.getCanSpawnHere() && this.getBlockPathWeight(new BlockPos(this.posX, this.getEntityBoundingBox().minY, this.posZ)) >= 0.0F;

and:
In entity/passive/EntityAnimal: #82 (EntityAnimal class):

    public float getBlockPathWeight(BlockPos pos)
    {
        return this.worldObj.getBlockState(pos.down()).getBlock() == Blocks.GRASS ? 10.0F : this.worldObj.getLightBrightness(pos) - 0.5F;
    }

So instead of light level > 8, what it needs is brightness > 0.5F, which is achieved with light level > 11, which stays in contrary to the statement in EntityAnimal#122

Video with explanation:
https://www.youtube.com/watch?v=sm43geXAgys

Please and Thank you.