Definitely midland, OP is in Indiana (only midlands in IN). Pattern doesn't match Eastern or Western.
Algae itself won't bother the turtle, but these guys are big on basking so if there is algae on the shell it means they aren't basking which causes lots of problems (algae and fungus grow on the shell, inadequate lighting which leads to Vit D3 deficiency, which can cause soft shell/metabolic bone disease). Not to mention that if the basking area wasn't proper most likely other aspects of care are not either.