Don't ever use chemicals to clean a turtle's shell. Learn to clean a turtle's shell with tips from the owner of a reptile store in this free pet care video.
My name is Tim Cole, I'm with Austin Reptile Service, I've been keeping reptile amphibians for over forty years and we're here to talk about cleaning turtle shells. And, the first thing that comes to my mind is why? Because naturally, in the wild, turtles do have algae growing on their shells. And one of the things that helps them do is camouflage. And it might even attract some smaller fish that want to eat the algae which, in turn, could be a food source for the turtle. So, granted, it may not look nice, it's a natural thing and I wouldn't worry too much about it, I wouldn't try to take it off. If you use any cleaning chemicals on a turtle you might actually do more harm than good. One of the ways you could get algae off a shell is if you dry dock a water turtle for a little while, just kept him out of the water long enough for that to dry up. But, that would make him uncomfortable and I think the best thing is just leave it and don't worry about it. If you have algae growing on the shell on the turtle that your keeping in the house, that's a good sign that you've got good lighting on your aquarium.