Can't be Skylar's dad. She states clearly that he's in prison for selling weed. I was thinking the same thing until I remembered that she tells MC that in the park. I think the Luna thing is just that Luna remembers him from the diner and she's scared of him because while people can look very similar, it's impossible for people to look exactly alike (excepting twins) without very extensive plastic surgery. Also, she says she never forgets a face, implying that she might have a photographic memory.
That's a fair point. I had thought of that. It just seems to me that Skylar and MC are close enough that she would have told him that the man in prison isn't her bio dad, and Luna is close enough to Skylar who is an adult and probably would have told her that her "dad" isn't her biological father. Speculating way too much. lol *shrug*