What is It.

[barry c]

these things won't get any bigger than an inch. they behave like an anemone, they eat, move and split the same as anenomes do.

[ABreefkpr]

Could be a type of mushroom. Does it flatten out at all Deb. If it is a mushroom it should cover it's base. If it grows a stalk first and then spreads out well Maybe Tony has something there with the Xenia theory.

Keith

[EmilyB]

The thing is I have hairy mushrooms and although in some ways it resembles them, it behaves much differently.

It looks exactly the same a week later as it did above. And turns up in a new spot pretty much every day. I've seen it detach and go travelling.

[Van down by the river]

Hello Emily,
Your picture is blurry, so it's hard to tell.Tell me if I am right: It has a cream overall color. It has small clusters of greenish "grapes/balls" around the perimeter? If so I've had this before.

It is a Thalassianthus aster

It's a small colonial anemone, it reproduces quite easily and can become a problem in a well fed tank. treat it the same as Aptasia.
I had one, then two, then three, you get the idea. I don't feed that tank heavily so it's population hasn't exploded, but if it got more brine or mysis I could see it easily taking off.