What is that

on his back? This should video not still. It looks like some kind of seaweed but it was moving slowly like a worm. One end of it was under the feathers of Tern but the rest of it was moving all the time slowly. Wierd.

Very hot. Heavy thunders in the morning near the south-west coast, some dark clouds above us at afternoon but then again blue sky and hot. + 34 degrees on my sunny back garden.