Can anyone tell me what this is?

[frags7]

So my 6 line went over the overflow and ended up in a tank that is just holding water at the moment. I have this tank covered in a foil insulation to stop heat loss, well when I pulled it back to look for my wrasse I saw this anyone know what it is? They are stringy kind of look like roots... this tank gets no light.

 

[Mrbh22]

looks scary to me! does it move?

 

[Jaz78]

I had something similar in my tank but smaller with the same pattern. There were about a dozen, i never noticed any issues as a result and eventually the disappeared.
From Google i came to the conclusion they were hydroids, yours looks similar.

 

[thegrun]

It's a sponge.

 

[viper12775]

cryptic sponge

 

[FuGu2]

might be a type of sponge