You have to be more specific--what did this mother-of-pearl looking substance come off of? The turtles? Something in the tank? What does it look like--is it flat, round? Is it hard, soft?
"You become responsible, forever, for what you have tamed." -Antoine de Saint Exupery-
Is the object thin, translucent and the shape of a scute/part of a scute? If so, it sounds like your one turtle may have shed one. How do their shells look when they're dry? Do they appear healthy?
"You become responsible, forever, for what you have tamed." -Antoine de Saint Exupery-
Your turtle's shell is divided up into individual regions. These are called scutes.
Do you have, or have access to, a digital camera? A picture of your turtles shell would be a great help. Sounds like your turtle just shed a scute which is perfectly normal....
The shell where the scute came off should look healthy--no fungus (cottony-looking substance), nothing "strange" on it. The area will most likely be a better color. The shell should be firm to the touch.
"You become responsible, forever, for what you have tamed." -Antoine de Saint Exupery-