White cacoon looking pods. What are they??

[skooby]

I have just noticed white cacoon looking pods on my live rock. At first they were open on the top and know I have noticed they are closed . They are the shape of a jeenie bottle . I dont know any other way to explain them. They are white, kinda dirty white. WHAT ARE THEY????

 

[brian'sreef]

they are probably a type of sponge i have a similar species in my reef they have grown a lot around the filter system areas and on the back wall where current is high.. i'd say that's what it is.. maybe someone else can help more.. a pic would be cool
Brian

 

[slb243]

I had a few on them on live rock (fish only tank)and they seemed to be multiplying until I purchased a raccoon butterfly. Then they all were gone. That was the happiest day of that butterfly's life.