Originally Posted by Socratic Monologue

They need more Vitamin A in their diet, likely.

Usually a good supplement such as 'Repashy Calcium Plus' on their food at every feeding is sufficient, but if they show those symptoms, using 'Repashy Vitamin A Plus' according to label directions will help. Turtles whose symptoms are severe enough to cause them to not eat normally can be dosed orally with castor oil to get them Vitamin A.

If this gets bad enough they can lose an eye (probably to secondary infection), so keep on top of this. I rescued a couple triunguis and went through this; they recovered, though one lost one eye. After hibernation they also sometimes need a little Vit A boost, though increased supplementation in the fall helps this.