Is This A Live Rock Hitchhiker?

[tekisui]

I added this live rock today, and noticed what looks to me to be an egg sac. I don't know what it is, if anything. I would appreciate any help in IDing this.

 

[tekisui]

I added this live rock today, and noticed what looks to me to be an egg sac. I don't know what it is, if anything. I would appreciate any help in IDing this.

Also--It is squishy and it doesn't move.

 

[seffieuk]

it's a lovely little sponge

Seffie x

 

[TigerIssey]

Harmless, I have had one for months.