sponges will grow on dead skeleton and rockwork. and ide agree that the frag you have is well dead.. that substance looks abit to dense to be sponge tho and sponge wouldnt of grown while you were at work unless of course your on a tour of duty ; )
sponges are good filter feeders and considered a good edition to your tank. have you poked it? is it soft and porous ie like a sponge.. or is it slimey? it could be a mold/fungus? its difficult to say from a picture alone.. your best bet is if it grows quick then take it out : )
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