It's hard to tell from the pictures, but looks like it could be a fungal infection. I would recommend frequent water changes. If the condition doesn't improve I would try dosing the tank with Kanaplex or another brand of kanamycin.
Can a fungus really pop up that fast?? That’s crazy! It legit happened overnight. I’ll go pick up either of those fungicides just Incase things take a turn!
It may not be anything yet, so you could just keep the water clean and see if it goes away on it's own.
I'm not sure how fast a fungal infection can develop, but if the fish cuts itself that can be how it starts. A round of kanamycin treatment isn't too hard on the fish, but definitely follow the instructions and stop treatment for a week after the 3 doses, (6 days) if you decide to try it.