This is a stauromedusa. It's about 1 cm across and attached to a piece of red algae (Mazzaella sp.). We think it's a species of Haliclystus.
Here's a view from the side:
I don't have many resources with me, so will have to do more research on this species when I return to California. For now here are a few more pictures of this wonderful animal.
The next image shows the stauromedusa holding a crab zoea (larva) that it had just captured. You can see the crab zoea's eyes and its spine pointing backwards. I don't know how often stauromedusae eat crab zoeae, but this one caught at least three while we were watching it today.
I first wrote about stauromedusae in November, so click here to review their anatomy and a little bit about their biology and read more about a different species that we found on Bodega Head last fall.