collage of two cropped photos

On a warm sunny day, turtles bask on half-submerged logs. This is a collage of two photos.
Above: about 40 turtles on one log!
Bottom: two red-eared sliders (Trachemys scripta elegans). The carapace of the right tortoise looks unusual: its left side looks covered with silt, like that of the second tortoise, while the right side of the shell looks "washed" of silt. The shell of turtles consists of two layers, bony below and horny above. Probably, the horny layer of the back of the shell is missing for some reason. It seems to me that a detached piece of horny layer is visible near the head of this turtle. I did not find any information whether the change of the plates of the horny layer is the norm, or is it a manifestation of some kind of disease.
Never before have I seen such a thing. Does anyone know an explanation for what is happening?