what is this? seems like a type of snile.

i think i know what these are, i think it starts with a c
but they only come out at night and they are creepy

one of them is like 2 inches

anyway should i keep them, are they ok to have or do i need to kill them for the safety of my reef.
sry if you can not make out this first pic i tired my best at 2am.

 

Stometella snail..Harmless

 

ok but what do you do. do they clean?

and thanks for the id zoan

 

They are a good part of CUC

 

Algae on the glass,rocks.

 

They eat algae, and very well.

 

ok thank you everyone. i wish i did not pull one out of my tank, dam. now i'm said i killed that thing.