What is this?!?!

[Scubalife]

I've had this sponge-like "thing" growing from under my rock since the day I started my tank. I wasn't too concerned about it till it started spreading threw that rock and onto another. What is this and how can I be rid of it? I would appreciate any input.

[betamed]

its a harmless sponge. brush it off with a tooth brush if you don't like it or it starts to overgrow a coral.

[tebstan]

aww, don't kill it. It's beneficial, a healthy addition to your tank. And the uglier specimens will politely stick to the dark side of rocks.

[Scubalife]

If it isn't harmful and in fact beneficial I'll leave it in my tank gladly. I just get nervous when things "spread". Especially if I don't know what it is. Thanks for putting my mind at ease!!