Its a simple bug. When I was messing around in Minecraft 1.9 I hit a shulker with a spectral arrow and he turned white. I could see him through blocks!

Put the summary of the bug you're having here

What I expected to happen was...:
Nothing really. Just trying to kill the shulker and this happened!

What actually happened was...:
I was shooting the shulker to see how long I could go without dieing and the glitch occurred

Steps to Reproduce:
1. Go to create and get the spawn egg
2. ... Go to survival and spawn them.
3. ... Shoot the mob with a spectral arrow and witness the bug for yourself