I've never heard anyone describe dead skin cells as excrement. I try not to be a prescriptivist, but I'm not sure grey water is a legitimate usage.
I don't understand it either, but using a word that most often means "fecal matter" tends to make things less attractive.
How many trans people do you know? Has a trans person gotten mad at you for something?
Charitably, it seems like people who have never met trans people seem to think they're balls of tinder, waiting to explode at any imagined slight. There is a lot of misinformation and propaganda out there and trans people are a punching bag.
Yes, words change a lot, and maybe someone might ask you to use a different word. That's not an attack on you.
Yeah, you might meet someone on a bad day or there could be a misunderstanding, but, in general, treating people with kindness and respect works out pretty well.