Flat, Round Black Creature

[Giacommodore]

Hello all,
I just got a one year old established 20 gallon cube two days ago and was looking in my tank a bit ago, went to the kitchen to get food for the tank, came back and there is a flat, round black creature attached to a rock.
At first, its edges fluttered but it is now completely plastered to the rock and there is a white slit in the middle that opens and shuts periodically. I was also watching a black antenna come out of the top of it and feel around on the rock. Mind you, this thing is completely flat.
I've never seen anything like this. Does anyone know what it is?

 

[ISpeakForTheSeas]

Elephant Snail/Slug (it's basically a big limpet):

Given the white patch in the middle, probably an Elephant Slug (Scutus sp.) - most are good to have, so it's probably a keeper.

 

[Giacommodore]

Elephant Snail/Slug (it's basically a big limpet):

Thank you! I ended up finding this out from my local fish store shortly after posting here