Time-lapse of some sea creature

Finally found a good use for a time-lapse video. Filming took about 20-minutes using Instagram's Hyperlapse. Hyperlapse's stabiliser is a little bothersome, as it kind of makes the video drift a little, even if the camera has not moved.

Now, I am trying to identify this creature. Do you know what this is? (This was shot at Fukue island, Japan or 福江島, if that helps)