What's this stuff?

Anyone know what this juvenile Pearsonothuria Graeffei is walking on? It's at Wori near Manado in Indonesia. I think I've seen pretty big colonies of it, and I have a feeling it stings. Maybe I got stung by some of it once.

http://www.oceanfootage.com/video_clips/NH08_814

Hi Nick - It's a brown algae, either Padina or a related species. Padina doesn't sting but it could be overgrown by something like hydroids that do.

Thanks Leslie, that'll be it!