I've seen the episode in question and the Kaled mutant isn't per se, melting. It simply has no substantial skeletal structure to support its mass, so, like the Martians in H. G. Wells' famous alien invasion novel, it surcomes to gravity. Production-wise, The goop is just there to make the thing more repugnant. Some fan theories suggest it's simply a lubricating slime to keep the creature from drying out and crack its epidermal layers, rather like a slug's mucous. Its pale color is due to an intense shaft of sunlight shining upon it for the first time in its hate filled existance. Can you tell I'm a "Doctor Who" fan? ;-) Sincerely, Bill

Tempt the Hand of Fate and it'll give you the "finger"!