The pipe fitting does look as if the bottom is 'open' so I dont think it is a rock - also there is a 'knob-type' thing on the top of it too.. The other 'plastic' thing looks more like a giant clam shell that we have here on earth.
http://marsrovers.jpl.nasa.gov/gallery/all/2/n/523/2N172797616EFFABGZP0695L0M1.JPG Unless it's a rock with flapping ears! I'd like to know what's covering the junk. Vegetation?