What is this?

[monza]

Nice sponge, in the book Reef Invertebrates it also mentions that most sponges like moderate lighting so if you just moved it you might want to make sure it's not getting to much light. If you could not see it before, most likely it was in a low lighting spot or even shaded.

Dave

PS. That book Reef Invertebrates is a great book, just got it and powered though it. .