What the...?

[fs40reef]

I'm tearing down my tank today to drill for overflows and returns. While taking out the live rock I noticed a couple of pieces with these dark brown patches. These dark spots are only on rock in the back of the tank where no light reaches. Can anybody ID please? Whatever they are they scratch off with a finger nail. I just want to know if I should be concerned.
Thanks,
Will

[hobogato]

hard to tell from the pic, but i would guess they are sponges

[fs40reef]

Ace,
They are flat and a little slimy. It almost looks like someone was painting cheetah spots on my rock. No reason to be concerned? They have apparently been there for a while.
Will

[CoryDude]

Ace is right, I think those are a type of sponge. I've had them all over my rocks on the undersides in low light, like you mentioned. Had them for years and years and they seem pretty harmless.

[txav8r]

Sponges would be my guess as well.

[fs40reef]

Thanks. I'll put away the scrub brush.
Will

[JoshOdphi]

yea looks like sponges to me...sponges are natural filterers...