weird jelly creature that splits brown stuff

[qxcontinuum]

Ok, so here it is. I've been seeing movements every since i've gotten this live Fijiji rock.
Today i've saw it, it is like a head with a mounth, has some tentacles coming from the month, eye on the sides. When a fish came around it has splitted some brown stuff 3 times and was hiding fast.
What could be?
Snail without a shell?
Don't say it could be a Sepia?
I've tried to pictured the best i could;

 

[qxcontinuum]

i think it is a snail without shell

it moves like one;

 

[d.french]

stomatella snail, there good to have and breed fast in aquarium which is good cause there good tank cleaners.

 

[Bifferwine]

Yep, I agree. Stomatella.

 

[dcantucson]

I just discovered one of these in my tank a couple of weeks ago. They are kinda weird. Looks like a curved finger nail.