Please tell me what this is.
 

is about the size of a quarter, very green, and was discovered in my reef tank after shifting some rock. thanks.

The name of the pic pretty much says it all. Bubble. Algae. :)

Thanks...is it ok to just leave alone?

No, I'd pull it out without popping it.

it partly depends on your definition of "ok". If by "ok" you mean it won't kill anything in your tank, then ya, it's ok. However, if by "ok" you mean it won't reach plague proportions and totally take over your tank then no, it's totally not ok. :biggrin:

Take it out with out popping it.. suck it out with a hose like you would when doing a water change. If broken it will spread! :twised:

I've read that this isn't true, however, to be safe, I'd try not to pop it. Sometimes it's unavoidable though, so don't stress if it pops.

But just in case..haha