Ah, I understand.
Sorry, Greg,
That was a long time ago and, to be honest, I had clean forgotten that you had posted it here. It was also one of the "good" images of defrosting slopes that I collected for my study of Sisyphi Cavi.
So, returning to polite mode, ....
The bright white part of the image IS dry ice - which looks not dissimilar to normal H2O snow and ice. When finely crystalline, it is white, but it can also form "block ice" like ice cubes.
We can tell that it is dry ice from the temperature - IR measurements show that those parts of the image are at ~-145 K, the equilibrium temperature of dry ice at martian pressures...