A sim I made in CAS(not alien but has light blue skin tone) married a townie and tried for baby. Neither has been abducted, nor had anything to do with aliens and she just gave birth to an alien baby. The baby's even wearing the alien outfit. What gives? Is this a bug?

Are you sure that the townie's not a half-alien? They can have half-alien children (half-aliens come in the normal bassinet, but have weird skin tones [usually] and wear the alien outfit as a baby).

He looks like a normal sim to me and has no one in the family tree. How can I find out if he's half-alien?

As far as I can tell, just wait for the baby to turn into a kid.

Once (s)he's a child, if (s)he can disguise themself, then it's a glitch. If the kid is some weird colour, but can't disguise themself, the kid is a half-alien, so the parent must have been a half-alien.

As far as I know (by searching forums and my own experience), a half-alien is the product of an alien/human, half-alien/human, or half-alien/half-alien couple. They can't use ANY alien powers, including disguise, but they are weirdly coloured, come in normal bassinets, and wear alien clothes as babies. They have hair and normal eyes.

An alien is the product of an abduction, alien/alien couple, alien/half-alien couple, or half-alien/half-alien couple. It's an alien.

Ok. thanks. The baby has a light greenish-blue skin tone though.

Sorry. I didn't mean to write "purple." I changed it.

I aged her up to child and she looks normal now, normal hair, eyes, clothes and now has the same skin colour that her mother has. So I guess it was some kind of glitch?