Can someone ID this thing?

[Mavric]

Looks like a bowl on the end of a stem. Noticed it while I was looking for GHA during my initial cycle today. It was just wiggling around with the flow so I thought I’d ask.

 

[1WildBill]

Cup coral?

 

[cristata.reef]

Can't be a cup coral due to the lack of corallites. I'm thinking some sort of bowl sponge