Sponge or Moldy thing?

[kubes]

I posted on here a while back with crappy pics of what I suppose is a sponge, well I am back now I bought a new camera and got hopefully better pics of what this is. If you could let me know what this is if you know. There are also little shrimp and copepods living in this little thing as well. It looks tanish-green, has a thin membrane around it, and has formed on one other spot in my tank. It looks very pourus, which makes me think it is a sponge, but at times it looks like a mold or something. Any help would be much appreciated.

 

[DaFrog]

That's a sponge

 

[SnowManSnow]

yup sponge

 

[wade1]

The "membranes" are siphons... where water passes into the organism so that they can filter out their dinners. Not bad to have as cryptic filtration in any tank.