White Worm on Rock?

I found this white worm looking thing on one of my live rocks the other day and I have no idea what it is! Thursday I looked in and there are HUNDREDS of little ones all over the tank walls!!!! I have no idea if they are good or not and I want them gone! If anybody knows please TELL ME!!!!!

 

I see pineapple sponges. And you may want to consider replacing that tank.....thats quite a chip taken out of the corner. I wouldn't be able to sleep at night knowing it was damaged.

 

+1 on pineapple sponges---- they are safe and usually come and go on their own.

 

Thanks guys! They were really scaring me I though they were some kinda predator or something. And as for replacing the tank I will as soon as I get to money too... its been like since before I set it up for saltwater but its not a big problem i.e. no water leaks and it isnt deep enough to get to the stuff that holds the walls together so its fine for now