Luca's mouth is totally back to normal now, thankfully. I'm so glad it turned out to be nothing more serious. He was in quarantine for 3 days, and he had a salt dip each day. Already after just the first salt dip, the mouth issue was much improved. I had to go out of town over the weekend, and the cycle is much stronger in the main tank than it was in the QT tank (even though it was seeded with plenty of cycled filter media), so since his mouth was looking so much better anyways, I put him back in the main tank on Friday morning before I left. I worried about him all weekend of course, but when I got back home, he looked completely back to normal!
As usual, he was super obsessed with zooming around foraging for food on the tank bottom while I was trying to take photos of his mouth.
It bothers me that I don't know what the problem was, but I'm mostly just glad that it's been resolved!