Alien sphere ID please

[cwatkins]

Hi all,

I don't have any pictures of this, but a description should be okay. If not I can get pictures tonight.

For the past few months we've had this round rather shiny silver ball growing on a rock in our tank. It's now about an inch in diameter.

It has a very little bit of coraline growing on it now, but it still appears to be growing. And I think it's soft because it's now hitting a branch of a coral, and it appears to be indenting versus growing around it.

At first I thought maybe it was a snail or something, but now I'm wondering if it's an algae of some kind?

It's only one single ball. No others anywhere else.

Thanks.

P.S. If an alien or predator is going to pop out of this thing and eat me, please let me know sooner than later! That way I can suck it out with the siphon