ID on this worm-like mass under a rock?

[bryanfuel]

This isn't actually my tank but was from a post on reddit that stumped me a while ago. Any ideas?

 

[P-Dub]

This isn't actually my tank but was from a post on reddit that stumped me a while ago. Any ideas?

Looks more like an alga than a worm to me.

 

[reef lover]

Sponge

 

[ahiggins]

I second the sponge. Harder to tell out of water but I once had a tube like sponge that looked sort of like a long tentacle anemone tentacle lol long and stringy.