What is it

[kumba63]

I just set up tank 3 weeks ago. I got some nice live rock and this was on it. Can any one tell me what it is.
Thanks

 

[pokmijn]

looks like a striped mushroom

 

[melosu58]

Maybe this

Saltwater Aquarium Corals for Marine Reef Aquariums: Green Striped Mushroom

 

[abushr]

Yep striped mushroom....i have several. Beware....they can go nuts and spread. In my sm tank the population doesnt seem to spread....but they love my big tank! I actually have thinned them out with an exacto blade when they decided to interfer with other corals. I've even used aptasia poison on them. Lol.