With a toothbrush try and gently scrub and wash off some of the algae, I think it may actually be a red eared slider. It looks like it may have the "ear" but it is very faded. Also where are you located?
Not a RES but it looks like a yellow bellied slider, which isn't native but I think they've been introduced there. Someone else that knows more about the northern species will hopefully be able to tell you definitively what it is.