While doing a rift on my DH i encounter on 2 separate floors, 1 enemy that would follow me around and just kill me whenever i stood still. I could fully see it following me and sometimes it would attack my pet, but pet did not interact with it at all. I could not see anything when ever i moused over it. It was a moon clan enemy for both cases.
From the stickied Known Issues thread at the top of the forum…