I think it is very important to follow your gut. If this guy made you feel uncomfortable, then maybe there was a good reason. I have been followed on my road bike by creepy men in cars. When it happens, I stop, text message the license plate number to my son with a just in case, look them straight in the eye, and inform them that I have just reported them to the police, and that they need to back off. I don't mountain bike so I am not sure the best equivalent thing to do, but obviously phoning either home or the park police are good options.