I noticed the same thing, tomorrow afternoon im just going to remove each snail and clean them off, I olny have 12 so it shouldnt be bad and I'll just continue to do so until i dont see em anymore.
I also just read an 8 line or banana wrasse is good for this also not sure if these can go in the same tank as a six line........
Here is a good article on pyramid snails vs rissoids which are beign but similar looking. In my tank I believe I have rissoids which come out at night and I have never found any of them on my snails or clam
I have a six line and he kept my clam clean. Unfortunately, he cant get to the snails when they are on the glass. But for the most part he does a good job.
the second pic is definitely some type of leather. They are too small to give a 100% ID as to what kind of leather. Maybe a close up of them would narrow it down but they appear to be doing ok.