Weird white fan like things growing on my live rock?

I just cleaned my 55 gallon saltwater tank a few days ago, changing all the water. I generally rinse my live rock in freshwater to kill algae. But today I noticed these little fan like things on the rock. There's hundreds of them. This tank and rock are not new. I've had for 3 ish years. Never seen this before. What are they? They react to touch by hiding their tentacles. Are they dangerous?

 

Looks possibly like aptaisa.

Very dangerous. Remove the whole rock IMO.

 

Agree with above... Looks like aiptasia.

 

Is this tank a FOWLR? It does look like aiptaisia anenomes which can multiply quickly and can sting and kill corals.Rinsing your live rock in fresh may not be a good idea as it will kill alot of your beneficial bacteria.

 

i'm a little confused, these whatever they are are small white in color and you say the rock is 3 years old, nems will not stay in limbo for 3days much less 3 years. so if there is no new rock been added, i just don't understand now there are so many just poping up so, please give use a better picture of them, so i can say they are nems for sure.

 

Picture is very blurry some better pics would help.
With that said, from what I can make out I do not see aiptasia. Google hydroids or colonial hydroids and see if that matches anything

 

I can't see anything in the picture. Does it look like this:

 

Or this:

 

I also think they are hydriods

 

I'm going with hydroids too