From a distance it does look like a slider (not sure about the red ear) but close up picture would help as I can't see crap any more! Even with my reading glasses. This part of getting old sucks!
Can't really tell from that picture, but the only native California turtle is the pond turtle (Actinemys marmorata), and it's NOT a pond turtle, so whatever it is, it shouldn't have been there in the first place.
Pretty sure its a Red ear with washed out colors. Male red ears also loose most all their color as they age.
Could possibly be a close cousin - the Cumberlain slider. Trachemys scripta troosti Identification wise - very similar to Red ears but the enlarged stripe behind the eye is yellow or orange instead of red