Not at all uncommon in older, faded individuals. The quality of the water in which they're living can also cause the characteristic red ear to be grimed over or stained. This is why it's important to look at the features as a whole when identifying instead of focusing on one thing. You also can't make out any yellow "s" shaped mark on the side of the head as is typical in scripta scripta.
While I believe this animal to be more dirty or stained than anything, melanism is fairly common in scripta elegans; I've never seen a melanistic scripta scripta. That's just one more reason for me to lean towards red eared slider as my guess.