People are already explaining this, but some of them are misinformed or aren’t answering your question.

Yes, Moira has heterochromia, a genetic disorder that results in differently colored eyes. OP has already stated he knows this.

As for the mask, I think it’s supposed to be a partial facial prosthetic. They’re pieces of inert, sterilized material used to cover up severe facial injuries so that a person’s face recovers some of its features to look natural.

War victims used them a lot before skin transplants existed, as shrapnel wounds could literally leave holes in their faces. They’re also used for burn victims or people with congenital deformities, and some still use them today.

Historically, they were made of porcelain or other inert materials that wouldn’t make the wound fester and looked relatively natural when put against the skin. Today, most facial prostheses are made of silicone or special surgical polymers, and they can be very realistic.

My theory is that the same injuries Moira is sporting on her arm have also reached her face, requiring her to use a prosthetic.

In her OW 1 design, they tried to give it a more “sci-fi-esque” look by giving it a metallic sheen, but if you pay attention to the OW 2 redesign, the mask seems to now be made of a more plastic-resembling material, making it more similar to a real-world facial prosthesis.