It is a zoanthid from the genus Parazoanthus growing on a sponge.
I just want to clarify that the sponge may very well be a Trikentrion sp. but the white growth is a zoanthid. Sponges do not have polyps
PRODUCT DESCRIPTION:
Aussie Tiger Sponge ~ Trikentrion Flabelliforme Yes those are Zoanthids.
I'm sticking with the cinnamon rolls vote on this one...
there is a guy on ebay selling it as a aussie spider sponge.
Aussie Spider Sponge LIVE CORAL! lps,acan, zoa,ricordia - eBay (item 120463683704 end time Sep-03-09 20:32:03 PDT)
hope this helps all