If you dont know what it is dont say anythingg

[Screwgunner]

Found this guy in my tank . I was told it was a black flat worm get it out. So I take it out . It is not a worm . Sea slug ? Then I'm told good for tank.

 

[MoreReef]

Looks like a black nudibranch... look at elephant slugs...

 

[blaxsun]

So is it back in the tank?

 

[Screwgunner]

Yes I took it out and it seems to have a hard shell on top so I know it is not a worm.

 

[Screwgunner]

Black limpet they have a shell. Algea eater.