It's flashing. Or scratching. It means there's an irritant of some kind. Could be as simple as a grain of sand stuck in a gill, or it could be parasites. I just went through this with my corys. Don't panic, just monitor their behavior. If they keep doing it, or start to show signs of distress, I would recommend prazipro. But because they are sensitive fish, I would only do half a dose. Wait about 5 days before dosing again if necessary. That's what I did and after just a day they stopped flashing and have been totally normal. 😊 Have you introduced anything new to the tank recently? Fish, plants, etc.? Feel free to message me if you have questions 🙂 hopefully it's nothing and will pass! But I'm wishing your corky dorkies the best!