Is my computer being controlled?
Applications like Little Snitch can be very useful for figuring out if your computer has been compromised, as they can detect incoming and outgoing connection attempts.

I would say that most likely the appearance of an unexpected person on Skype is just due to a spammer or scammer adding people at random from a list of names or email addresses. It's also possible that it's a case of mistaken identity, where the person on the other end is trying to contact someone with a similar name. Be wary of social engineering attempts, and do not open any links or attachments from this person! 

If the person is bothering you or you're just weirded out, you can block the individual by right-clicking their contact portrait and choosing "Block (name)...".