Right now it just looks like a partial leftover shed or irregular shed. Keep an eye on it, if it starts turning yellow then you should get him to a reptile specialist for a skin scrape to be analyzed.
I agree that it just looks like an area about to shed. Look for darker yellow or skin that looks wounded/scabby. At the moment it just looks like dulling skin about to shed though.
It's possibly a stain, but the even pattern and the dull color look like shed to me. You could give him a bit of a soak in some warm water to clean him up a bit.
I just set Swordtail's timer for his bath and paused it so I could actually fill his soaking bowl up and he crawled over my phone and canceled the timer
Mirage came out of brumation on April 26. He was doing great. On May 2 he started acting funny. We just redid his tank, and he keeps going into one of his hides. He just lays there. He shows no intrest in food. HELP!