Is that coating WP? I found out on the Sipsey trip recently, to my surprise, that WM blue CCF is NOT waterproof. I had a blue pad strapped on top of the MMP, and we hiked through an hour or two of (mostly light) rain. While setting up camp under the tarp under the now pouring rain, I realized that part of the pad had absorbed a fair amount of water. I don't know if other pads might be more water resistant. So a sack might be a good idea for trail use, in addition to storage.
I just ran to Wal-Mart (only like a quarter mile from me) and the pads are advertised at 100% waterproof. That was on ALL of the pads they sell, not just the blue ones.