2002 Chevy Silverado Heater Core Hose removal

For a Chevy 2002 Silverado, 57000 miles, 5.3L

I am trying to remove the heater core hoses at the firewall. There are 2 different style hose clamps. The one on the right is a standard metal "squeeze clamp". The one on the left is some kind of plastic ring that I am not familiar with. It is round, black plastic, about 5/8" long, and has 2 holes in the side at 180 degrees apart from eachother. The hose can be rotated around on the heater core tube. I tried turning it, twisting it and pulling it straight out. My question is - How do I remove that hose connection? Any help would be much appreciated.

Thanks
:)