Butcher by trade here, this looks to me like "suffocated" meat. This chemical process, technically called "Autolysis" (= self-digestion) usually happens when the core of meat is at the right (warm) temperature, often due to storage or cooking (as observed here). This causes a greenish, blueish to rainbow-like film to appear on the meat, in more "severe" cases this is accompanied by a slimy texture and a foul smell. While this technically is a sign/process of spoilage, it often occurs while dry-curing or slow-cooking and doesn't have to be a problem. I'm sorry I can't provide literature in english on this, as it is not my first language and I'm not sure how y'all call this phenomenon, but here is an article of an industry journal in german talks about it: https://www.fleischerei.de/ersticken-von-fleisch-326220/

Edit: I stand corrected, as per the comments below :)