That is a type of protein and connective tissue.
Mainly you have collagen and elastin in a cut of meat.
Collagen turns into gelatin through heating and melts away. The elastin will get softened. I believe what you’re seeing is the elastin.
The best authoritative answer I could find specifically concerning the food safety was from a chemistry professor and science columnist. According to him, there is no known danger from consuming food that has fallen victim to the 'lasagna battery' effect.
Personally, I have had this happen numerous times and served and eaten the offending food with no ...