Need some help identifying something

[bryanleemcbride]

I have these long single stranded white strings on my live rock. They don't look like spider webs. Any idea what they could be?

 

[rssjsb]

Looks like a sponge. Did you upend this rock recently? I've had these in my tank before but always in shaded areas.

 

[DEFCLONE80]

I find these in my tank time to time. Do you have a sand sifter cucumber?