Entities spigot 1.8.9

Hey guys,
I have a question about Entities in minecraft (spigot).

Code:

Slime slime = (Slime) p.getWorld().spawnEntity(p.getLocation(), EntityType.SLIME);
slime.setSize(1);
slime.setCustomName(ExpManager.getLevel(p.getUniqueId()) + " §6Level");
slime.setCustomNameVisible(true);
//((CraftSlime) silverfish).getHandle().setInvisible(true);
p.setPassenger(slime);

I want to create a Slime. Give him a Custom Name which is visibel for everyone. Hide the Slime and set the Slime invulnarable.

My Problem is that the Slime is Visable, his Customname ist only visable for the Player who has the Slime on his head und the Slime can be damaged. How can I fix these thinks?