I would love for this to be large colonies of photosynthetic microorganisms. These vanish in the martian winter and return in the martian summer! At the moment, most scientists hypothesize that they're CO2 geysers. I'm no geologist, but so far, I don't know of any geological events that positively correlate with seasonal change. Anything is possible, though! It could be a CO2 thaw, but I'm surprised the vapor isn't pure white. We've all seen dry ice, right? It's white and sublimes into a beautiful cloud-white vapor.
My only question on the geyser theory is this: Where are the shadows? If these are giant plumes, shadows would be visible emerging from their bases. Shadows are detectable on large natural and man-made objects from Google Earth, so where are the shadows here? (yeah, you can tell i'm really hoping it's microbial life, can't you?)