Unkillable rare mob below terrain

Hello,

multiple times I saw this mob below the map so I was unable to damage it in any way, but it still chasing and attacke me. Luring it up to a lower terrain also did not work.

For now I just ignored that, but this time on the "sump" map it was spawning as rare preventing me from completing the map:

https://imgur.com/a/rBa972H