It's been a while but I could start to explain it and maybe someone can pick it up from my rusty memory.
Mercury thiocyanate, Hg(SCN)2, decomposes at about 165 degrees Farenheit[1] to elemental components. I would guess that the shape of the product in that video is most likely due to the gases that are formed (most likely cyanide and mercury gas). I'm guessing the products are sulphur, cyanide vapor and mercury vapor. I'm also guessing that there are some oxides of sulphur and nitrogen forming since oxygen is readily available.
Mercury thiocyanate, Hg(SCN)2, decomposes at about 165 degrees Farenheit[1] to elemental components. I would guess that the shape of the product in that video is most likely due to the gases that are formed (most likely cyanide and mercury gas). I'm guessing the products are sulphur, cyanide vapor and mercury vapor. I'm also guessing that there are some oxides of sulphur and nitrogen forming since oxygen is readily available.
[1]: http://cameochemicals.noaa.gov/chemical/1048