«to create (some) fake rotten skin» - 'some skin' «de la peau», therefore «créer de la fausse peau pourrie».  Remember 'some' is often left out in English but can't be in French.

https://kwiziq.learnfrenchwithalexa.com/revision/grammar/du-de-la-de-l-are-used-to-express-some-or-any-partitive-articles