How Knov apperance kept on changing?
His appearance changed because he was so terrified.  The idea was that he was so scared of the beings they were exposed to that his hair went white and he rapidly lost weight.

Stress can cause rapid weight and hair loss. I don't know if hair turning white from fear is real but it is a common belief (see crash test dummy's MMM). That is what you are seeing exaggerated here.  What you are supposed to note as remarkable is how he continues to aid the group and stick with it even though he is so debilitated.  He is more scared but somehow the bravest GD person there.