A riddle of an invert

[chrisnif]

So,
What floats on the surface of the low flow fuge, is about 1 cm x 0.5 cm rectangle, pulses to swim (like a jellyfish, but it swam downward), when it gets on the rock it moves like a very fast slug. Oh and i touched it (dumb bass me) and it kinda tingled. I know some of my live rock gives me a tickle if i move it, but this was something that made me jerk my finger away.
I don't think it can fit through the impeller into the DT, and i dont see it having the ability to swim against the flow in the U tube to swim against the current into the DT.
I'm more or less just wondering what is this thing. I though it was a dead slug, but slugs dont sting or pulse

 

[spanko]

Can you take a picture, or draw a picture so we can see better what it looks like?

 

[shrimpy brains]

Hydroids??????? Pic would def. help.

 

[chrisnif]

well it looked like a slug, brown not grey, it was rectangular, and like i said it pulsed when touched, almost like it were a jellyfish, and then it acted like a slug on the rock. it was about the size of a pinky nail like 5mm x 10 mm not round but very rectangular.
Thanks again

 

[fretfreak13]

Sea hare? I saw one in the wild and it kinda did that, but didn't sting when I picked it up. I don't even know if they can or have been kept in captivity, but that's what your description sounds like to me.

 

[shrimpy brains]

Flatworm?? Lots of people keep sea hares. I've never seen mine swim.