What is on Phoenix’s Leg?

What is on Phoenix’s Leg? I cannot stop looking at this animation. The first frame was taken on the 8th sol (a Mars day) and the second around the 31st sol. As you can see from the animation, it seems that some mystery material is either growing, moving or multiplying around one of the legs of the Phoenix lander. The most likely source is frost building up on what would be a very cold surface, however the only issue is that it doesn’t look exactly like frost and it hasn’t appeared on any of the other legs.

Image note: We added an artificial fade from 1 frame to frame 2 and scaled the original image to around 300%. Due to artifacts from increasing the scale and compression we added some noise to smooth out the overall appearance.