Some people have been saying the word "crud" since it has a same meaning for shit or sometimes crap (depending on if it's a swear word or not). So, I'm wondering if the word "crud" is considered a euphemism for the word "shit".
It just means any sort of unpleasant or unwanted substance. Thus, it could also cover excreta of all sorts, as well as soap scum, dirt, greasy residues and so on. It also applies metaphorically to other things, as many words describing physical substances do.