Hi, this is my 1 y/o red-footed tortoise hatchling Ruby. When I took her out to soak I saw these white markings on the bottom of her shell & wanted to make sure if it was shell rot or not. Here is a picture
It's hard to tell if it's a start of something. Keep an eye on it and if it gets any worse or changes put some athletes foot cream on it. In the mean time, be sure to keep the top layer of substrate dry. Wetting the buttom layer by pouring the water in the corners and a wood chip, like orchid bark as a top layer will help keep the tort off the wet stuff