In consequence, I went digging through another edition of Galen from the 1580s. In this book, I found something that with my poor Latin skills I'm not 100% sure is the right recipe, but it looks like it may be a better candidate for the original cerate/cold cream. Maybe the recipe originally known as "cerate" was something else, and this mixture took on the title for lack of a better name? I'm transcribing it here as best I can, though it uses a few non-standard shortcuts in how its written.
Demonstratio, qua ostenditur, aquam frigefacere ex cerati, quod ex ea conficitur, effectu.Porro q[?] aqua ois dulcis perspicuo frigefaciat, hinc quoq; didiceris. Ceratu humidum si ex aqua frigida subactum acurateq`; madefactum calido alicui affectui imponas, protinus ipsum frigefacies. Oportet autem qua plurimum admiscere ipsius aquae, mistioq`; hoc maxime modo optime perficitur. Ceram quatum fieri maxime potest purissima, oleo liquantes humidu facere ceratum oportet: deinde refigeratum, ac rasum, in mortario manibus subigere, aqua effundendo frigidam quantu nimirum ceratum accipere valet, ac nondum aqua circunfluat. Id refigerat et eos qui in febre uruntur, si hypochondriis superponatur.
Near as I can tell, it's a recipe for wax scrapings beaten with fresh water in a mortar, maybe with oil, used to cool fevers and other "hot" ailments.
Why was this recipe not in the other book? It may be the 1530 edition was copied from a faulty manuscript, or perhaps even the 1530 edition itself has lost pages over time. It also may be that I simply missed it (again, I don't read Latin very well, especially this very old fashioned Latin that Galen uses.) I had located this by running a search for the word "aqua" in the manuscript, and if the word was illegible in the other edition, the computer might have overlooked the passage.
So that this point, it's looking like maybe two recipes from Galen were combined -- his skin-soothing rose oil cerate, and his cooling water cerate. Sounds good.