No, milk is not bad for cast iron.
Milk and Cast Iron: A Closer Look
While milk may not be the ideal choice for cleaning your cast iron, it won't harm it. The milk proteins can actually help create a protective layer on the surface of the pan.
Milk for Cleaning Cast Iron
Milk can be used to clean your cast iron, but it's not the most effective method. Milk's cleaning properties are mild, and it might not remove stubborn food particles.
Milk for Seasoning Cast Iron
Milk is not recommended for seasoning cast iron. The milk proteins can create a sticky residue that can interfere with the seasoning process. Stick to oils like flaxseed oil or grapeseed oil for seasoning.
Conclusion
In summary, milk is safe for cast iron. It can be used for cleaning, but it's not the most effective method. Avoid using milk for seasoning.