As far as I could gather the digestion relies on enzymes which in turn set requirements for pH.
Is it possible for an alien creature to have a different set of enzymes that require basic environment and use lye as a base to provide such environment?
I suppose it would also need different mucus to protect its stomach from digesting itself.
The creature in question lives in earth-like environment in caves. It has slow metabolism so it does not need a lot of food. It's food mostly consists of remains of other dead animals. It can also scrape some minerals from the cave walls if it's useful. When threatened it produces toxic gas to protect itself. Digestion process can be very slow.
I looked up more info about mucus and it seems that soap that is created when fats mix with lye should not be much of a problem as mucus is a gel, not oil.
Though the process of digesting lipids remains messed up.