It is quite normal for plecos to rasp on a snails shell. There are microscopic algae treats lying on them. The pleco is not really trying to harm the snail in any way.
I have apples and plecos and ramshorns and plecos together and I kinda like watching the snails stop walking when the plecos come over to "clean" their shells.
Oh, the fish won't bother the snail at all.