Well, most aquatic turtles grow algae on their shell. It is only natural that algae would use a turtles carapace as a host(I don't meanm this in a parasite way). In the wild the algae help the turtlwe camoflauge. But in captivity it is recommended to scrub it off since it will make the turtle more prone to shell rot and fungus. Also it sounds like you have a cool pond. Where do you think it came from. And how big is your pond?
:I have a baby painted turtle that I caught in my pond. It is only about 2 inches. It has algea growing on the top of it's shell why does it have algea on it.