1 - Some demons are visable by humans, hence why I made the post earlier about the women and her child. Weak demons are visible in the forms of humans or an object, like the plant that took hold of the girl, and strong demons can become ethereal and some can change their appearence. Demons can see other demons, and some humans are gifted with the ability to see invisible and formed demons.
2 - Yes it's probably at first going to be OMG WTF IS THAT KID RUNNING FROM? But as i described the demons becomes more visible the stronger it gets so soon it will become visible to Seth and co.
Yuki has the ability to sense the presence of invisible and formed demons but cannot see them like some people can btw.