What is It.

[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.