The skin taken from McCains face turned out to be non-cancerous.

Melanoma's first big attack on McCain was back in 1993. It was able to launch the attack after it had received funding from the sun, which is a source of Ultra Violet rays.

McCain was on the run from the serious cancer and was doing was doing his best to launch his own counter insurgency. It was invasive and it was working, then the melanoma launched a brilliant plan to deploy 30,000 more melanocytes into the region known as the face. "The surge" was said to be working and the melanoma claimed it was
winning. And it was, but now...

The melanoma developed plans to withdraw but wants to stay to make sure the skin stays safe from other cancers. The Melanocytes unveiled plans to put three permanent moles on his skin. One on the shoulder and two on the face.

Melanoma claims the attack on his skin wasn't about the oil, but we see how dry his skin is.