I recently noticed that the base of my alligator snapping turtle's tail has turned a bit pinkish white. I might be overreacting but I need to be 100% sure it's not a sign of a disease like shellrot. Here's a picture...
that is from sand getting in between his shell and tail and rubbing it. It can lead to an infection but it's not there yet. Swap out the substrate and keep an eye on it. You should be fine though.