Dunno, but I don't think it's HDPE.
Not really that flexy, and doesn't feel like it's self lubricating (HDPE leaches oils continiously, making the surface perpetually slick -- good for reducing friction, bad for adhesives). The shiny you're seeing is the finish off the injected mold.
It does have a a slight bit of flex to it, but I'd say it leans toward the harder side of plastics without going brittle hard.