What is this growth?

[CatRiderF7]

I emptied a tank yesterday and I removed a rock and found this on the bottom of it. It was sticking to another rock with whatever this white growth is.

<a href="http://s565.photobucket.com/user/chriseisenbraun9/media/growth.jpg.html" target="_blank"><img src="http://i565.photobucket.com/albums/ss93/chriseisenbraun9/growth.jpg" border="0" alt=" photo growth.jpg"/></a>

[MARINECRITTERS]

Sponge, harmless.

[ReeferBill]

Could be a cone jellyfish. Does it feel spongy or slimy?

[guidedbyechoes]

Sponge, harmless.

was harmless.

[CatRiderF7]

Its more spongy but relatively firm.
Just something I can leave and not worry about it taking over the tank or anything?

[julie180]

Sponges are good filter feeders. They tend to like low light.