To me, "film" in this sense implies either a different substance or a noticeably different form of the same substance. "On the top of the porridge was a thin film of porridge" sounds silly. "On the top of the porridge was a thin film" could mean a film of some other contaminating substance, or could mean a film that originated from the porridge but had a different consistency (for example, some ingredient had separated out).
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Mr Wordyor could mean a film that originated from the porridge but had a different consistency (for example, some ingredient had separated out).