Cotton looking strings growing from my live rock?

[obriene2]

Hey guys! I have what looks like white cotton growing from my live rock? I have never seen this before and wanted to see it anyone knew what this is and if I should be worried.

This is a new 29g Biocube I set up about 2 weeks ago. Thanks so much!

 

[Dav3]

Looks like sponge growing