Wish I could help but I've not come across something that looks like that. A white stringy sponge... hmmm, anything's possible in saltwater, lol. Anyhow, I agree with @DutchAquarium and have used the list linked above to identify a critter or two that had myself and many others pretty stumped.
I'd also ask that you please keep us updated if and when you do identify it.
If you continue to have trouble, go ahead and tag me on this thread and I will post tags for the big guns so we can ask for their help.
Thanks. For the reply’s. I assumed it was a sponge when it was growing out of that one spot. Now I’m uncertain because all over my rockwork I’m beginning to see strands of it starting to pop up. It is not tentacles because they do not move or retract, it is getting larger though so it is growing. I spent a good 20 minutes scrolling through that link above and saw a bunch of useful stuff.