What is this?

It's taken about 90 days from when it first became detectable, to what you see here. This mass of life is growing directly beneath rock, which keeps it almost entirely out of direct tank light.

When it first showed up, it was white. It didn't have the clam-like or sponge-like siphons at first. It became grey with time. I knew it wasn't eggs, as my only inhabitants are 2 banggai cardinals, which are mouth brooders, some snails whose eggs show up as ribbons on the glass, and some hermits.

My first guess was sponge, but I thought sponge was photosynthetic.

 

Nope, sponges are filter feeders and that looks to me like what you've got

 

Its a beauty!

 

You'll get tons of odd shaped sponges if everything is going good. I used to get yellow, loved 'em.

 

YUP.......sponges.......sponges and more sponges.;D

 

Thanks folks. I guess sponge it is then. They're really fascinating when starting to grow. Pristine white at first. Now grey. I've got hundreds of them in high-flow areas. They're all over my sump, growing in my refugium, I've got a lot of life teeming around everywhere. Part and parcel with reef tank ownership. Great stuff!

 

So cool isn't it, everywhere you look something is growing whether it be in your main display or your sump. There is a dark area in my sump that is now home to some interesting looking filter feeders. You gotta love this hobby!

 

Looks more like tunicates to me because of the elongated finger like branches with the typical tunicate like intakes. Also, it's semi-transparent like color, and your initial statement that it began as a clear mass and not a colored mass also furthers the likelihood of it being a group of tunicates and not a sponge. Sponges are generally colorful (although not always) but sponges usually appear and begin to grow with the same color that they end up as at any size. I've had several tunicate colonies in my reef tank, usually appearing as clear to begin with, and slowly growing with size, develop a greyish hinge to it as it grows. But hey, thats just my guess. I've yet to see a sponge look like though.

 

Phayes, I stated the mass began white in color, not clear.

Many people interchange white for clear and vice versa.

I googled tunicates in "google images" and I don't believe what I have is a tunicate. Out of a few dozen pics, I saw one that somewhat resembles what I have, but my specimen doesn't match that variety of tunicate.