I have had these on my live rock for a while and they move around the rock.
This is one up close that I got a good picture of awhile back then didnt ask.
So what is this? Shaz
it shrinks in size almost to nothing then grows again....the movemnt is so slow that it takes days to weeks for it to move 1/4 of an inch. right now they are super tiny maybe 1/8th inch balls. This one when I got the pic was almost 1/4 inch ball without the strings attached or the tentacle things... They are colored white to the greenish blue color of the one pictured, you can see a few smaller ones on the rocks in the picture.. Shaz
well right now they are small and look like the smaller ones in the pic surrounding the bigger one. The move super super slow and dont always have thier strings out in the air like the big one did in the pic. BTW these are in the 12 gallon eclipse with the crappy lighting...Shaz
When I first looked at the picture the first thing that came to mind was a urchin. But I never heard of them shrinking and growing back. I would just keep a eye on them.
very cool~ I would be guessing some type of sponge...and that IS a guess... I have lots of wierd looking sponges in my tank that are totally bizarre...