A sponge? An egg case? What the?

[reefgurl03]

Ok, so I am new at this. Everyone started somewhere and I am perplexed. I got some nice fiji a few days ago (cured) and plopped it in my tank. I noticed an opaque filmy-looking thing in the crevice of one of the rocks. It does not move, and it remains this bluish-white color. I thought maybe an anenome? This morning when I peeked in my tank there was a large slit down the middle of it, but I couldn't see anything moving or anything new in my tank. Throughout the afternoon the slit has gotten smaller, and now it is a small circle (about 2-3mm in diameter). I wish I had a picture but don't have a digital cam. Any ideas? It's very strange. The whole "thing" is about 10-15mm long, and has a soft mushy appearance. Thanks in advance!
Bente

 

[Len]

WELCOME TO REEFS.ORG!

It sounds like either a sponge or a tunicate to me, but it's really impossible to say for sure. Likely, it's nothing to concern yourself about (I can't imagine anything resembling your description that is dangerous). Live rock has such amazing life on them, huh?