That looks like a quick coupler. If you push the outside ring towards the grill, you should see a clip at the end of the inside piece, It should just be a round circle clip. If you take a small screwdriver and lift that out of the groove and remove it, the outer collar should slide off, the balls that hold the hose in will fall out, and you should be able to grab the inside part with pliers. Again, it is propane, so it is RIGHT to loosen.