Strange Growth on Rocks

I need some advice. I have a growth on my rocks that I believe I should be removing, but I'm not sure. It has the same shape as a mushroom, but it isn't. It is greyish-white in colour.

Does anyone know what this is?

It doesn't appear to be a Discoma. It does not have the same texture as a mushroom. It is about as thick as thin cardboard. Also, they have spread. It almost looks like a wide mouthed megaphone, kind of has the same shape as a mushroom. I know it sounds strange, but it is hard to discribe.

I don't know the names of things, so anything people recommend, I will look up on the net and try to match it by appearance. If I had a digital camera, I would post a picture.

Any more suggestions.

It isn't soft at all. In fact, I don't believe it is a coral. It does not react to light or to the touch. It doesn't move at all.

I actually was thinking that it may be a type of algae, but I cannot find any pictures on the net when I search for algae photos.

Very close to the first, but your web links brought me to look up their site. The attached link is what it is...

http://www.globaldialog.com/~jrice/algae_page/padina.htm

Thanks for your help today. If you are familiar with this, do you know if I should be worried about it?