Terraria Ghost :O Help MEh!?

So, to all of you who play Terraria, I have been exploring caverns and for a while, in the darkness BEHIND me, I see these strange footprint like things appear and disappear. I don't mind them but it is very strange. Then, sometimes I find dismembered glowing Zombies in the rock. They glow and disappear. I was wondering if you knew what this is or if it is a bug. These either show up VERY RARE or very often. Thanks for asking :D