I have two of these in my tank. Unfortunetly, I was unable to upload a video I had taken but, I’ll try to explain it best I can.
a dark mouth comes out of the white and casts a web. It looks like it “reels” in the web with its mouth. I’m assuming it does this to catch food in the water. The white seems to be just a shell. It never moves and seems like it’s the same size. Although, it was not there when I first got my live rock. Any ideas on what it could be?