While walking on maps with monsters I keep seeing these "ghost" monsters, they are just walking around and you can't fight them or anything, is this a bug?
Those are shadow monsters. It is perfectly normal since the Guardian system was put in.
I don't think they bother you so don't mind them. There is a skill that allows you to fight them I believe. I don't know which. I don't care much for Guardians so I haven't really looked up those skills.
Alright, Thanks :)