Ever notice how there's so much focus on the eyes with some characters? Particularly ones with a "final form". Sans' eye glows, Undyne the Undying's eye glows, Mettaton's NEO's eye flashes a tiny bit (and in some unused frames for Mettaton EX, the same eye is shown to be in construction, meaning maybe Alphys only finishes it when you start killing monsters). Even Gaster is shown with a melted eye of sorts.
I always thought it was the result of DT injection of some sort. Gaster tried it on himself early on and failed, but it worked when done to Sans. Undyne's was administered in a rush to stop you and her form couldn't hold it, but Mettaton's artificial body was sturdier and did the job.
But who knows, it's probably just Toby being a shit