What do you think this is?

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I noticed it a few days ago. I thought maybe a tentacle, but it stays out just like this all the time. I would think if it were a tentacle at some point it would retract. It is about 2" or maybe a little more long. Clearish white and flexible.

Most likely a spionid worm. Isnt harmful.

if it hasent moved its probably a type of sponge.. i had some pop in my tank a while back from a piece of LR i got.. the back side was covered in what looked like strands of cotton.. actually it look more like..really thick mucus blowing in the water but they went away...or something ate um either way you should be fine i wouldnt worry about it. if it spreads you can always remove it/cut it back or scrape the rock off

check out the these and see it its similar.. http://www.wetwebmedia.com/SpongePIX/SpongeIDF11.htm

I also think it is a sponge. It is growing and still in the same spot. I should take a video of it since the pic that I showed looks like it is spread across the rock when in reality it is anchored only at the base and the rest is standing straight up off the rock. It moves around from the power head hitting it somewhat but does not appear to be moving on it's own as it is very rythmic to match the power head.

It doesn't look like any on their page. I have looked there before as well. Thnk you guys though!

yea, and there thousands of types of sponges lol so finding a pic of one...non commercial sponge can be quite difficult