id needed please , what is this white thing ?

[TrickyVicky]

can anyone please help with identifying what this is on my rock , i added this piece of rock about 3 weeks ago now this white thing was like a tube and has now since opened up in half im not sure if its trying to seperate or grow or move , it hasnt moved anywhere since i added it but i have seen it close up in half and look like a tube and then open back up again .
i have look all over the net and cant find anything it is a little smaller than my finger nail .

any sugestions ?

 

[TrickyVicky]

ps im sorry if the picture is bad , its the best i can get at the moment .

 

[GerritM]

My guess is an mushroom (Discosoma)

 

[scuba guy ron]

looks like a sponge to me

 

[TrickyVicky]

thanks guys , its a kind of see through white , with little pink flicks in it ..

 

[coralsfriend]

Looks like white sponge.