So, I wanted to ask this because this started to happen quite recently while I never had this issue before. I initially thought it was them just having a cold because this generally start around winter but this keep happening in other seasons as well and I'm pretty sure they didn't become vampire while I was playing with my families. Some of them become normal again but others keep that grey skin. They also have that same grey-ish skin in their portraits. Is it a glitch?
(sorry for the blurry screenshot, I don't know why but the game thought it was a thumbnail but you can still see the landlady having a different skin from the other sims)