If you put the transparent part of the slime in front of a door ( i used spruce) you can see right through the doors to outside.

Steps to reproduce:

1. Spawn a slime in a room of doors
2. Place any block outside then close doors
3. Look through the transparent part of the sime and the doors will be invisible but you will see the block of choice 

Observed results : the doors are invisible and you can see outside

Expected results: when you look through the transparent part of the slime you will see only doors and the block of choice is not seen

Edit: effects glass panes. Will be investigating more

Effects : glass panes, doors