I have jellyfish looking plants growing out of control on my live rock How do I get rid of whatever they are?

Answers:
Actually, they are probably hydra. I had some once. Here's a pic http://www.nature.ca/rideau/b/images/hdr. and another http://www.fishieforums.co.uk/picupload/.

They won't harm larger fish, but you can take the rock out and scrub em off. By the way, they are relatives of the jellyfish :D Neat, huh?

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Algaecide.

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Ask Sigourney Weaver. She did a good job on creatures like that in the Alien movies.

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it was probably algea. and i think it got worst and now its a fungus. get an algea eater fish and your problem should be solved. and there's tablet for getting rid of algea u can try that too.