"Sponge" gave me a clue, so I started looking at pictures. It may be a pineapple sponge, but all the pictures I found of both tube sponges and pineapple sponges had a distinct opening at the end. Mine does not. It looks like the business end of a big bumblebee, but all white.
Yep, distinct white thing at the very center. My phone is the only camera I have at the moment, so no better picture available.
Probably a ball sponge then if so completely harmless
Yes. It (or something like it) was in a dark corner of the liverock about a month ago. That one is gone now. I guess we could take a guess based on the lifecycle of ball sponges if this is a descendant or if it's something different that's mobile.
The marine creatures are so many. In older tank I had for about a year some sort of snail, but instead of hard shell it was covered all together with sponge of some sort, none was able to ID it, the snail was growing and the sponge on it was growing like the snail shells. 100% there wasn't hard shell because I watch it few times under and behind it strong light, I was able to see all the stuff in this snail.
