It sounds like you want to have your slime make use of Chitin:
This document: Chitin and Chitosan
gives a strong indication that at least some forms of Chitin are resistant to the acids you are wanting to use. Interestingly the same document indicates it is susceptible to some organic acids such as citric acid...
OK - After some pointers in comments from the OP about what the desired actual outcome is:
This article here:
Extremely Strong and Transparent Chitin Films
Indicates there is a valid method to make use of chitin in the way you want. Unfortunately the detail is locked away behind a paywall... This could be a method of harvesting Chitin in large quantities from the slime - and manufacturing thin panes of "glass".
As an alternative, I suggest that having your slime develop a thin flat transparent Chitin in response to particular stimulae may give you the outcome you are looking for.
So have your farmer discover that if he traps the slime in a space with a flat surface, and "feeds" it corn husks and cobs, it will develop a hard (ish) skin that remains when he starves the slime. After cleaning the dead slime away, he is left with a semi transparent sheet. Possibly he would also need to shock it with electricity, to stimulate the growth of the "armour". Or maybe subject it to poking with sticks? (So some kind of device that lowers a large tray of densely set bristles)
Different feed-stocks may produce different clarity of the resulting "glass"