Any idea what this is?

It is actually a type of mushroom, but its common name is "white ball anemone." The scientific name is Pseudocorynactis caribbaeorum. They like dark places as you can see yours in a crevice. They will open up at night and eat anything that sticks to their tentacles. They reproduce by pedal laceration -- in my experience pretty slowly. Fortunately, this type of Pseudocorynactis stays very small (biggest I have seen is about 1/2" across). If you have a blue flashlight, they have pretty neat fluorescence.

Awesome, thanks!

I had one that showed up in my tank with some zoanthids. It didn't seem to bother anything and reproduced to make a pretty little garden. I let it be.